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

mtwebster/cjs,pixunil/cjs,mtwebster/cjs,pixunil/cjs,mtwebster/cjs,mtwebster/cjs,pixunil/cjs,pixunil/cjs,mtwebster/cjs,pixunil/cjs