Revert "getParameterNames is really a const function"

This reverts commit 3979e3d91d4b16a29b9bbe2288e54e27c732f1e4 due to the fact it
breaks CMSSW_7_0_X_2013-08-04-0200. See:

https://cmssdt.cern.ch/SDT/cgi-bin/showBuildLogs.py/slc5_amd64_gcc472/www/sun/7.0-sun-02/CMSSW_7_0_X_2013-08-04-0200/