Remove MFPageActions/MFEnableSiteNotice vars
The MFPageActions and MFEnableSiteNotice config variables were
deprecated in Ib64b9400 (19/02/2016) and can now be removed.
Now that MobileContext::singleton isn't invoked in the extension
registration callback, add a strong warning that it shouldn't be in
future (see I8d57f485 for context).