vm: rip out ObjectWrap from ContextifyContext

This was a remnant of the original Contextify code, wherein
ContextifyContext was a user-exposed object. In vm, it is not, so all
of the ObjectWrap and function-template stuff for the ContextifyContext
constructor is now unnecessary.
vm: use MakeWeak to fix leaking contexts

This is always something you should do when using `SetHiddenValue`,
apparently. Fixes #6115. Thanks @tjfontaine for the tips.