Remove deprecated ruby implementation of find commits
#1443 (closed) moved the find commits rpc to be pure go. Once deployed, we no longer need the ruby implementation
- ruby/lib/gitaly_server/commit_service.rb#find_commits
- ruby/lib/gitlab/git/repository.rb#raw_log
- ruby/lib/gitlab/git/repository.rb#log_by_shell
part of this task is ensuring that nowhere else in the codebase uses these methods
We also need to remove tests that hit these methods