EditPage: Remove legacy non-OOUI render mode

EditPage: Remove legacy non-OOUI render mode

Mostly remove it at least. Some things from the transition are still
left around:

* EditPage::isOouiEnabled() will now always return true and is soft
  deprecated as some extensions are using it to detect whether in OOUI
  mode.
* EditPage::getCheckboxes() is left around since getCheckboxesOOUI()
  calls it. Someone else will need to disentangle that.
* The "mw-editform-ooui" class is kept as some on-wiki JavaScript uses
  it to detect whether running in the OOUI mode.
* CSS is still using the .mw-editform-ooui class. It should be
  transitioned to use .mw-editform in a follow-up.

Other cleanup:
* mediawiki.action.edit now directly depends upon oojs-ui-core instead
  of transitively via mediawiki.widgets.visibleByteLimit. As a result, the
  mw.loader.using call was removed from the JavaScript file.

Bug: T172315
Change-Id: I2b468c8b846db015b5a1e3d2500abb8ea252c442