[libcxx] Remove custom CMake code targeting Mac OS 10.6
libc++ has dropped support for Mac OS 10.6 for a while, and we don't
have any testers set up for that OS.
This commit puts in an error message so that people can reach out to
the libc++ maintainers in case support for 10.6 is still expected (as
opposed to silently failing in weird ways). We can completely drop
support for 10.6 and remove the error message some time in the future
when we're sure that nobody is relying on it.