[libc++][NFC] Remove <__memory>

The header is unused now, so we can remove it.

Author: philnik777

libcxx/include/CMakeLists.txt

@@ -914,7 +914,6 @@ set(files
   execution
   expected
   experimental/__config
-  experimental/__memory
   experimental/__simd/aligned_tag.h
   experimental/__simd/declaration.h
   experimental/__simd/reference.h