Remove deprecated call to wfLoadExtensionMessages.

This was removed from core in I9b9ecc9818d17c98748f39177c28554f8c485de5.

I'm not sure where $loaded is used, so rather than removing it, I just
set it to default true.

Change-Id: I08adfa06db2a79c2fd179b033ba49e4310e1a554