Remove vcruntime._Private.xstddef

`<xstddef>` was removed since MSVC 14.37. It isn't meant to be used by most users, but the reference will break all clients of `vcruntime`, including Swift standard library itself.

Author: stevapple

stdlib/public/Platform/vcruntime.modulemap

@@ -707,11 +707,6 @@ module std [system] {
       export *
     }
 
-    explicit module xstddef {
-      header "xstddef"
-      export *
-    }
-
     explicit module xstring {
       header "xstring"
       export *