Removed mutable from L1GtTriggerMenu

The declaration of mutable on a member data was unnecessary since
that member data was never modified in a const function. Removing
the mutable avoids warnings from the static analyzer.
Merge pull request #3260 from Dr15Jones/removeMutableFromL1GtTriggerMenu

Multithreading -- Removed mutable from L1GtTriggerMenu
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X