[libc++][CMake] Remove unnecessary conditional for defining new handlers

It turns out that whether the new handlers should be provided is orthogonal
to whether new/delete are provided in libc++ or libc++abi. The reason why
I initially added this conditional is because of an incorrect understanding
of the path we're taking when building on Apple platforms. In fact, we
always build libc++ on top of libc++abi on Apple platforms, so we take
the branch for `LIBCXX_BUILDING_LIBCXXABI` there.

llvm-svn: 358616
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a4bd340bf5015169a7dcdff013f018a9338610c1
diff --git a/src/new.cpp b/src/new.cpp
index 43b234a..901e785 100644
--- a/src/new.cpp
+++ b/src/new.cpp
@@ -22,7 +22,7 @@
 #   include "support/runtime/new_handler_fallback.ipp"
 #elif defined(__GLIBCXX__)
     // nothing to do
-#elif !defined(_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS)
+#else
 #   include "support/runtime/new_handler_fallback.ipp"
 #endif