[libc++abi][libunwind] Don't override libc++'s handling of exception features

0e04342a simplified exceptions-related configurations for libc++abi
and libunwind by reusing the logic in libc++. However, it missed the fact
that libc++abi and libunwind were overriding libc++'s handling of exceptions.

This commit removes special handling in libc++abi and libunwind to use
the logic in libc++, which is the right one.