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

[jloser]

Summary

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.

[philnik]

LGTM, but let's see if anybody else has any comments. We should definitely get better at working through the TODO LLVM <version>s.

[jloser]

In D141345#4038479, @philnik wrote:

LGTM, but let's see if anybody else has any comments. We should definitely get better at working through the TODO LLVM <version>s.

Agreed. I'm in no rush to ship this, but it would be good to do it before we cut the LLVM 16 release soon (1/23 I think?)

[ldionne]

Nice catch, thanks!

The CI needs a bit of love right now but I think none of the failures are related.

libcxx/CMakeLists.txt

 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit multilib libc++. This option is not supported anymore when building the runtimes. Please specify a full triple instead." ${LLVM_BUILD_32_BITS})
 if (LIBCXX_BUILD_32_BITS)
   message(FATAL_ERROR "LIBCXX_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
 endif()
 
-# TODO: Remove this after branching for LLVM 15
-if(LIBCXX_SYSROOT OR LIBCXX_TARGET_TRIPLE OR LIBCXX_GCC_TOOLCHAIN)
-  message(WARNING "LIBCXX_SYSROOT, LIBCXX_TARGET_TRIPLE and LIBCXX_GCC_TOOLCHAIN are not supported anymore, please use the native CMake equivalents instead")
-endif()
-
 # Feature options -------------------------------------------------------------
 option(LIBCXX_ENABLE_EXCEPTIONS "Use exceptions." ON)
 option(LIBCXX_ENABLE_RTTI "Use run time type information." ON)
 option(LIBCXX_ENABLE_THREADS "Build libc++ with support for threads." ON)
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)