Use `remove_foreign_key :timelogs, name: '...'`

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9908#note_25324225

db/migrate/20170124174637_add_foreign_keys_to_timelogs.rb

@@ -49,15 +49,8 @@ class AddForeignKeysToTimelogs < ActiveRecord::Migration
     Timelog.where('issue_id IS NOT NULL').update_all("trackable_id = issue_id, trackable_type = 'Issue'")
     Timelog.where('merge_request_id IS NOT NULL').update_all("trackable_id = merge_request_id, trackable_type = 'MergeRequest'")
 
-    constraint =
-      if Gitlab::Database.postgresql?
-        'CONSTRAINT'
-      else
-        'FOREIGN KEY'
-      end
-
-    execute "ALTER TABLE timelogs DROP #{constraint} fk_timelogs_issues_issue_id"
-    execute "ALTER TABLE timelogs DROP #{constraint} fk_timelogs_merge_requests_merge_request_id"
+    remove_foreign_key :timelogs, name: 'fk_timelogs_issues_issue_id'
+    remove_foreign_key :timelogs, name: 'fk_timelogs_merge_requests_merge_request_id'
 
     remove_columns :timelogs, :issue_id, :merge_request_id
   end