mediawiki.toc: Clean up left overs

Follows-up d7a40c8 which creates the mediawiki.toc module and
moved the tocToggle method from mediawiki.util.

The tocToggle callback used to be a public method and therefore
had two things about that that are obsolete:

* Taking the toggle as parameter.
  This is a private method now, and we already have a reference
  to the toggle.

* Asserting that the #toc has an <ul>.
  There is no need to defer this check this far in, nor to repeat
  it on each click. The old code checked it earlier was well, but
  we used to repeat the check here as the method was public.
  Now that it is a private helper method we can simply return
  early before setting it all up in the first place.

Also:

* Removed jsduck-like construction. This file is being indexed,
  as part of src/mediawiki/, but turning this into a class seems
  odd. It doesn't really have any structure to it.
  It's just UI glue. Turned it into an ignored file (no doc blocks).

Change-Id: I21d4f735032b96f00291502e06d9d35a74648143