audio/media: Remove unused field
audio/media: Remove unused field

track_watch is no longer used for anything since MPRIS interfaces was
adopted.

pkarasev3/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pkarasev3/bluez,silent-snowman/bluez,silent-snowman/bluez,ComputeCycles/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez