more clang-tidy fixes:
clang-analyzer-deaddode.DeadStores in recurrencerule.cpp clang-analyzer-optin.cplusplus.VirtualCall in calendar.cpp, freebusy.cpp, incidence.cpp, memorycalendar.cpp readability-redundant-control-flow in freebusy.cpp readability-avoid-const-params-in-decls in memorycalendar.cpp