I didn't test FastSim (shame on me), but based on grepping ```muonCkfTrajectoryBuilder```  appears only in https://github.com/cms-sw/cmssw/blob/CMSSW_7_1_X/FastSimulation/Muons/python/L3Muons_cfi.py#L19, where a ```FastL3MuonProducer``` EDProducer is configured. However,
* the producer is configured only if a flag ```old==True```, which is ```False``` by default in the configuration file, and
* ```FastL3MuonProducer``` doesn't seem to exist anymore (nothing to migrate from ESProduct to helper plugin in here).

At the time I concluded that this piece of L3Muons_cfi.py would be obsolete, but didn't see the larger picture until now (i.e. if the "old" piece can indeed be removed, so could the import you noted).