Revert r49880, r49883, r49885 - add uniwiki/CreatePage extension to core

Revert r49880, r49883, r49885 - add uniwiki/CreatePage extension to core

This looks pretty funky...

  A page with the title Main Page already exists. Would you like to edit the existing page?

  Yes. I want to contribute to the existing page.
  Yes. I want to contribute to the existing page.

Lack of interlinking tools means that this would promote creation of orphan pages, and the UI isn't very good. Doesn't seem to be a clear way to disable it in favor of alternate creation methods either.