Remove green outline from `New branch unavailable` button on issue page

## What does this MR do?

- remove mistakenly added green outline from `New branch unavailable` button on issue page
- remove mistakenly added green outline from `Checking branches...` button on issue page

## Why was this MR needed?

`New branch unavailable` and `Checking branches...` buttons had green outline though that should only be added to the `New branch` button.

## What are the relevant issue numbers?

fixes #21006

## Screenshots

### Before

![before-checking](/uploads/f37f62c99602143d7ddc27fead4035c5/before-checking.png)

![before-new-branch](/uploads/4926f7c57b97831496c79b4176ad02a3/before-new-branch.png)

![before-unavailable](/uploads/041c32fa7eef86403c3ee749d4f1131a/before-unavailable.png)

### After

![after-checking](/uploads/918484eca0ccda2d27a8bb3e08a569fb/after-checking.png)

![after-new-branch](/uploads/ae380933d3e82117c887dabfb06a915a/after-new-branch.png)

![after-unavailable](/uploads/d861b3b379de404c62cf27a7207b005c/after-unavailable.png)

See merge request !5858