Fix in RM for files not in FC

Makes optional the change from Stephane about returning an error when a file is not in the FC (rm.removeFile). The default is still to generate an error, can be overwritten by an Operations() option in the CS DataManagement/IgnoreMissingInFC (set for LHCb to True).

Also avoid calling twice the FC (exists and getReplicas) in removeFile

Few simplifications in setting return dictionaries and remove unused variable definitions