launch-new-instances: Drop obsolete signals and callbacks

The 'launching' signal and this._onActivateOverride callback were
removed from gnome-shell in 7ecb5af587af7ed892c6cfc5af0858a2acb04905
and 7ecb5af587af7ed892c6cfc5af0858a2acb04905 respectively.

https://bugzilla.gnome.org/show_bug.cgi?id=721864