Fixed size of buttons in branches list

## What does this MR do?

The buttons on the branch list were using `btn-xs` this MR removes that so the buttons are a normal size to match https://gitlab.com/gitlab-org/gitlab-ce/tags

## What are the relevant issue numbers?

#18677 

## Screenshots (if relevant)

![Screen_Shot_2016-06-17_at_10.38.31](/uploads/19fc173e3f549a84bb14517613403792/Screen_Shot_2016-06-17_at_10.38.31.png)

See merge request !4735