Remove unused mixins

## What does this MR do?
Remove unused mixins

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Reduces unused css

## Screenshots (if relevant)
Before:
```
$git grep 'btn-big' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:107:@mixin btn-big {

$git grep 'input-big' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:97:@mixin input-big {

$git grep 'solid-shade' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:45:@mixin solid-shade {

$git grep 'shade' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:41:@mixin shade {
app/assets/stylesheets/framework/mixins.scss:45:@mixin solid-shade {

$git grep 'linear-gradient' -- '*.scss'
app/assets/stylesheets/framework/buttons.scss:310:  background: linear-gradient(180deg, $white-light 25%, $gray-light 100%);
app/assets/stylesheets/framework/mixins.scss:20:@mixin linear-gradient($from, $to) {
app/assets/stylesheets/framework/mixins.scss:22:  background-image: -webkit-linear-gradient($from, $to);
app/assets/stylesheets/framework/mixins.scss:23:  background-image: -moz-linear-gradient($from, $to);
app/assets/stylesheets/framework/mixins.scss:24:  background-image: -ms-linear-gradient($from, $to);
app/assets/stylesheets/framework/mixins.scss:25:  background-image: -o-linear-gradient($from, $to);
app/assets/stylesheets/framework/nav.scss:11:  background: -webkit-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/framework/nav.scss:12:  background: -o-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/framework/nav.scss:13:  background: -moz-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/framework/nav.scss:14:  background: linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/pages/diff.scss:167:        background-image: linear-gradient(45deg, #e5e5e5 25%, transparent 25%, transparent 75%, #e5e5e5 75%, #e5e5e5 100%),
app/assets/stylesheets/pages/diff.scss:168:        linear-gradient(45deg, #e5e5e5 25%, transparent 25%, transparent 75%, #e5e5e5 75%, #e5e5e5 100%);

$git grep 'border-radius-left' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:12:@mixin border-radius-left($radius) {

$git grep 'border-radius-right' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:16:@mixin border-radius-right($radius) {
```

After:
```
$git grep 'btn-big' -- '*.scss'
(nothing)

$git grep 'input-big' -- '*.scss'
(nothing)

$git grep 'solid-shade' -- '*.scss'
(nothing)

$git grep 'shade' -- '*.scss'
(nothing)

$git grep 'linear-gradient' -- '*.scss'
app/assets/stylesheets/framework/buttons.scss:310:  background: linear-gradient(180deg, $white-light 25%, $gray-light 100%);
app/assets/stylesheets/framework/nav.scss:11:  background: -webkit-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/framework/nav.scss:12:  background: -o-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/framework/nav.scss:13:  background: -moz-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/framework/nav.scss:14:  background: linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
app/assets/stylesheets/pages/diff.scss:167:        background-image: linear-gradient(45deg, #e5e5e5 25%, transparent 25%, transparent 75%, #e5e5e5 75%, #e5e5e5 100%),
app/assets/stylesheets/pages/diff.scss:168:        linear-gradient(45deg, #e5e5e5 25%, transparent 25%, transparent 75%, #e5e5e5 75%, #e5e5e5 100%);

$git grep 'border-radius-left' -- '*.scss'
(nothing)

$git grep 'border-radius-right' -- '*.scss'
(nothing)
```

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #21559

See merge request !6092