[libc++] Don't automatically define LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS with libcxxrt

libcxxrt is only used on FreeBSD and the FreeBSD CMake cache already
sets LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS to ON, as it should. This
avoids defining LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS redundantly in
a place where it doesn't belong.

Differential Revision: https://reviews.llvm.org/D152971