[libc++] Remove warning for `LIBCXX_SYSROOT`, `LIBCXX_TARGET_TRIPLE`, and `LIBCXX_GCC_TOOLCHAIN`

Support for these CMake variables has been a warning for a while now.  The
comment indicates to just remove the warning message entirely as anyone impacted
had to have update to new mechanisms in order to use `libc++`.  So, remove the
warning message.

Differential Revision: https://reviews.llvm.org/D141345