Revert "Workaround for Rails 5, where LIMIT..."

This reverts commit 6823e7de.

Originally implemented in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21839, but an
error was reported in
https://gitlab.com/gitlab-org/gitlab-ce/issues/51729 resulting in a
workaround introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21879.

Now Rails 5.2 is used, this workaround no longer seems needed, so this
reverts it.

lib/gitlab/database/subquery.rb

@@ -6,11 +6,7 @@ module Gitlab
       class << self
         def self_join(relation)
           t = relation.arel_table
-          # Work around a bug in Rails 5, where LIMIT causes trouble
-          # See https://gitlab.com/gitlab-org/gitlab-ce/issues/51729
-          r = relation.limit(nil).arel
-          r.take(relation.limit_value) if relation.limit_value
-          t2 = r.as('t2')
+          t2 = relation.arel.as('t2')
 
           relation.unscoped.joins(t.join(t2).on(t[:id].eq(t2[:id])).join_sources.first)
         end