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