[Runtime] Remove spurious include of <filesystem>.

This was added when Saleem suggested using <filesystem> to avoid having
to do lots of string manipulation, but it turns out we can't use it on
macOS because of minimum deployment versions, and we also can't use it
on Ubuntu 18.04 because it doesn't exist there.

The code related to it was backed out already, but I missed the include.

rdar://106183273

Author: al45tair

stdlib/public/runtime/Paths.cpp

@@ -21,8 +21,6 @@
 #include "swift/Runtime/Win32.h"
 #include "swift/Threading/Once.h"
 
-#include <filesystem>
-
 #if !defined(_WIN32) || defined(__CYGWIN__)
 #include <sys/stat.h>