Change ListModeBase::DoRehash() to not be virtual
Remove a bunch of useless classes representing simple core modes
Pass prefix rank and prefix char to PrefixMode constructor
Migrate ModeChannelOp and ModeChannelVoice constructors to builtinmodes.h, remove cmode_{o,v}

The default value for levelrequired is HALFOP_VALUE, meaning the ModeChannelVoice class is not required but it will be useful in case the default changes