Remove magic comments from Ruby files

## What does this MR do?

Remove magic comments (`# encoding: UTF-8`) from Ruby files.

## Why was this MR needed?

Since Ruby 2, UTF-8 is the default encoding and those comments are no longer necessary.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5163#note_13134307

cc @rymai 

See merge request !5456