build: remove unnecessary modification of CMAKE_REQUIRED_DEFINITIONS

This is unnecessary now that the flag handling has been fixed.  The flags will
be added properly in the main CMakeLists.txt after the config-ix inclusion which
performs the required check.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@203639 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed