Remove vector reserve in SeedGeneratorFromRegionHits

Removes a vector reserve that at 140 pileup massively overestimates the size required.  Removing it does not noticeably increase runtime.  Memory use and timing before and after were presented in the 05/Jun/2014 reconstruction meeting (https://indico.cern.ch/event/321540/session/4/contribution/13/material/slides/0.pdf).