Avoids critcial warning ('GLib-CRITICAL **: Source ID 10 was not found when
attempting to remove it') in case the applications initiates another glib
context/main loop.

The new gst_bus_remove_watch() is available since gstreamer API 1.6,
otherwise fall back to g_source_remove().

Task-number: QTBUG-69468
Change-Id: I4343865cb41a2d62ac4b0c0c04f60f13727f56d0
Reviewed-by: VaL Doroshchuk <valentyn.doroshchuk@qt.io>