Setup: Move MWDebug logic to MWDebug.php

Setup: Move MWDebug logic to MWDebug.php

* Remove checks in HTMLFileCache.php and Article.php.

  These haven't been needed since the same check was added to Setup.php,
  many years ago. When FileCache is enabled, The Setup.php code disables
  MWDebug. There is no reason for FileCache to then also disable itself
  based on unused config. That means both of them lose.
  We now handle this logic in one place: MWDebug::setup().

* In rebuildFileCache.php, turn it off explicitly, just in case.
  The previous code there didn't work because finalSetup()
  is called after doMaintenance.php includes Setup.php, which
  is what checked this config var to decide on MWDebug::init.
  On the other hand, it's also always off in CLI mode.
  But, let's not depend on that, maybe we decide to enable it on
  CLI one day! Just keep it off explicitly here.

Bug: T189966
Change-Id: I45a8f77092249751dc6f276aa5bb67ebf5b4f64c