Hide MediaWiki's "Editing help" link, as we provide it in the toolbar

Ported from Vector extension's ext.vector.footerCleanup module, but
with less hacks. Depends on change Id9269876 in MediaWiki core.

Also remove unhelpful comments.

Bug: 43689
Change-Id: I36ecd06b6fc0cc5ce95bc43db303b1b542e6c81b
Merge "Hide MediaWiki's "Editing help" link, as we provide it in the toolbar"