mainmixer: If main port is removed set main port as none
mainmixer: If main port is removed set main port as none

Change-Id: I5d1baabea4f6d74e5e9712443fb578e9f76179ed

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