object: remove bogus comment and code about GC in a secondary thread

handle_toggle_up() is only ever called from the main thread, and
that's where GC marking happens. The background GC thread only
deals with sweeping, and not even of JS objects!

https://bugzilla.gnome.org/show_bug.cgi?id=725024
object toggle notify: do more work without deferring to an idle

The only case we really need an idle is when called from a secondary
thread (and even there it's arguable the idle is fixing anything).
Inside the main thread we're in full control of what the GC is
doing, and we know when it's safe and when it's not to touch the
JSAPI. Deferring to an idle while the GC is already in the sweeping
phase is late, by the time the idle runs the JS object is already
dead and we're accessing freed memory.

https://bugzilla.gnome.org/show_bug.cgi?id=725024
util: remove unused and semi-bogus API

Was removed in the last commit.

https://bugzilla.gnome.org/show_bug.cgi?id=725024
Use background finalizing for thread-safe stuff

For wrapper objects that hold thread-safe or unique C structures,
set the appropriate flag to send the finalizer to the background
thread, so that we don't block the mainloop more than necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=725024