compositemixer: Remove probe only if probe_id has a valid value
compositemixer: Remove probe only if probe_id has a valid value

This avoid to remove probe more than once

Change-Id: I4c40b07530165ddf435b44e46dfa74ba636b39f7

TribeMedia/kms-elements,shelsonjava/kms-elements,Kurento/kms-elements,Kurento/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,Kurento/kms-elements