Remove before_validation for old import sources

Once phabricator and gitlab.com are removed as import sources, and migrations added to remove these sources from the database record for ApplicationSettings, the before_validation check that manually removed either phabricator or gitlab from these ApplicationSetting.curent.import_sources, can be removed.

app/models/application_setting.rb

- before_validation :remove_old_import_sources


-  def remove_old_import_sources
-    self.import_sources -= %w[phabricator gitlab] if self.import_sources
-  end