Remove prev/next buttons on issues and merge requests

## What does this MR do?

Remove prev/next buttons on issues and merge requests sidebar

## Are there points in the code the reviewer needs to double check?

No

## Why was this MR needed?

The buttons were rarely used and added at least 1 query each on every page load which we could live without.

## What are the relevant issue numbers?

#18026

## Screenshots (if relevant)

Below is how it looks. Not sure if it's too much empty space. #17697 discusses a possible substitute for what to put there.

![Captura_de_pantalla_2016-06-06_a_las_11.09.26_a.m.](/uploads/7b379327aa6c608775f99bf47a9c3352/Captura_de_pantalla_2016-06-06_a_las_11.09.26_a.m..png)

![Captura_de_pantalla_2016-06-06_a_las_11.09.38_a.m.](/uploads/68bcae6754ed5ac48f1a6925fdd17d3f/Captura_de_pantalla_2016-06-06_a_las_11.09.38_a.m..png)

See merge request !4442