[libc++] Remove unused CMake option LIBCXX_DISABLE_MACRO_CONFLICT_WARNINGS (llvm#66249)

Thanks @Gadal for noticing.

Fixes llvm#63703

Author: ldionne

libcxx/CMakeLists.txt

@@ -302,7 +302,6 @@ endif()
 # about #include_next which is used everywhere.
 option(LIBCXX_ENABLE_PEDANTIC "Compile with pedantic enabled." OFF)
 option(LIBCXX_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
-option(LIBCXX_DISABLE_MACRO_CONFLICT_WARNINGS "Disable #warnings about conflicting macros." OFF)
 
 option(LIBCXX_GENERATE_COVERAGE "Enable generating code coverage." OFF)
 set(LIBCXX_COVERAGE_LIBRARY "" CACHE STRING