Remove some useless require_relative statements

Signed-off-by: NRémy Coutable <remy@rymai.me>

lib/gitlab/git/blame.rb

-require_relative 'encoding_helper'
-
 module Gitlab
   module Git
     class Blame

lib/gitlab/git/blob.rb

-require_relative 'encoding_helper'
-require_relative 'path_helper'
-
 module Gitlab
   module Git
     class Blob

lib/gitlab/git/repository.rb

 # Gitlab::Git::Repository is a wrapper around native Rugged::Repository object
-require_relative 'encoding_helper'
-require_relative 'path_helper'
 require 'forwardable'
 require 'tempfile'
 require 'forwardable'