[libc++abi][NFC] Remove some cruft from <__cxxabi_config.h> (#164578)

In `<__cxxabi_config.h>` there were a few things still around which
aren't ever actually used. This removes some of that cruft.

NOKEYCHECK=True
GitOrigin-RevId: 46a8ddbd1494812474502de5f9841492225c28a1
1 file changed