Stop using common IMTs in ReponseSpectra service response.
When making modifications to GMM web service responses to handle pga and pgv explicitly, we accidentally did away with an unused flag (it was hardcoded to false) that set the response spectra service response to report all IMTs supported by a GMM (instead of the set common to all GMMs requested; current behavior). When using modern MPRS GMMs, this doesn't make a big difference, but when adding older GMMs., the spectra range is limited. There may be issues with combined GMMs.