qtdemux: Don't stop task when resetting
This is a regression that was introduced in cca2f555 (yes, 9 years ago).
The only place where a demuxer streaming thread should be stopped is when the sinkpad is deactivated from pull mode (i.e. PAUSED->READY).
Attempting to stop the task in this function would cause this to happen when a FLUSH_STOP or STREAM_START event is received... which can cause deadlocks.