Review feature switches/Remove old always on switches

ChakraCore contains numerous feature switches that in practise are never used - e.g. there's a switch to disable promises and one to disable proxies etc. Some of these bloat the codebase AND require runtime checks for whether they're enabled or not.

EDIT: we should also consider removing phase/option switches that are never used see #4311 for one example (though in that case it's an always on switch)

We should investigate removing some/several of the older ones that are unlikely to ever be switched off and all associated checks.

Note, see lib/Commmon/ConfigFlagsList.h for all switches as the place to start looking.

edit: The PR ref'd below makes a start at this but there's more to do.