Remove default column from services table

After removing and ignoring the default column from the
code in a81914c7, we can now drop the database column.

changelogs/unreleased/217581-remove-default-column.yml

+---
+title: Remove default column from services table
+merge_request: 39817
+author:
+type: other

db/post_migrate/20200819082334_remove_default_from_services.rb

+# frozen_string_literal: true
+
+class RemoveDefaultFromServices < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      remove_column :services, :default, :boolean
+    end
+  end
+
+  def down
+    with_lock_retries do
+      add_column :services, :default, :boolean, default: false
+    end
+  end
+end

db/schema_migrations/20200819082334

+c12f3f5b76e1065867682216348dd95c22d605c30ae54615f2596b1d84aad199
\ No newline at end of file

db/structure.sql

@@ -15404,7 +15404,6 @@ CREATE TABLE public.services (
     tag_push_events boolean DEFAULT true,
     note_events boolean DEFAULT true NOT NULL,
     category character varying DEFAULT 'common'::character varying NOT NULL,
-    "default" boolean DEFAULT false,
     wiki_page_events boolean DEFAULT true,
     pipeline_events boolean DEFAULT false NOT NULL,
     confidential_issues_events boolean DEFAULT true NOT NULL,