gstreamer_backend: Remove extra buffering probe
This patch adds explicit removal of potential dangling buffering probe.
Not removing the old probe can prevent playback until the entire application is restarted.
The broken state could be reached by pressing the play button again, while some buffering was already in progress.
Closes #658 (closed)
Some issues sound like they might be somewhat related, in that the playback never comes back, once in failure mode.
Edited by Philipp Bartsch