Remove deprecated function mw.util.toggleToc

* Remove dependency from mediawiki.util to mediawiki.toc.
* Load module mediawiki.toc only when toc is existent.

Gadgets that use the messages "showtoc" or "hidetoc" should explicitly
load the module mediawiki.toc or use their own messages.

Follows-up I3ca2acb70db98d00e3f1b (implements mediawiki.toc).

Change-Id: If0438b7b6f4649434e2b83133d6f583f2f8eff16
Merge "Remove deprecated function mw.util.toggleToc"