Remove Users.support_bot column

This column is not present in `db/schema.rb` and hence needs to be
removed conditionally.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/66901 for background

changelogs/unreleased/ab-remove-support-bot-column.yml

+---
+title: Remove Users.support_bot column
+merge_request: 32554
+author:
+type: other

db/post_migrate/20190902160015_remove_support_bot_column_from_users.rb

+# frozen_string_literal: true
+
+class RemoveSupportBotColumnFromUsers < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    return unless column_exists?(:users, :support_bot)
+
+    remove_column :users, :support_bot
+  end
+
+  def down
+    # no-op because the column should not exist in the previous version
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 2019_09_02_131045) do
+ActiveRecord::Schema.define(version: 2019_09_02_160015) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"