Repository size is Gitaly only

Closes https://gitlab.com/gitlab-org/gitaly/issues/393

lib/gitlab/git/repository.rb

@@ -403,13 +403,7 @@ module Gitlab
 
       # Return repo size in megabytes
       def size
-        size = gitaly_migrate(:repository_size) do |is_enabled|
-          if is_enabled
-            size_by_gitaly
-          else
-            size_by_shelling_out
-          end
-        end
+        size = gitaly_repository_client.repository_size
 
         (size.to_f / 1024).round(2)
       end
@@ -1822,14 +1816,6 @@ module Gitlab
         commit(sha)
       end
 
-      def size_by_shelling_out
-        popen(%w(du -sk), path).first.strip.to_i
-      end
-
-      def size_by_gitaly
-        gitaly_repository_client.repository_size
-      end
-
       # Returns true if the given ref name exists
       #
       # Ref names must start with `refs/`.