Remove the definition of difftime

The build is broken after:
https://android-review.googlesource.com/c/toolchain/mingw/+/1350713

This is because difftime is defined as static __inline in the header file. We don't need a separate definition in .c file.

difftime.c is removed upstream in:
https://sourceforge.net/p/mingw-w64/mingw-w64/ci/19b6d96eb45ef1314e276f4ea6c3f2e0d7910120/
But this change is too big to cherrypick.

Change-Id: I40c14f703e7c0378efdbcb9693f92886f293ef03
diff --git a/mingw-w64-v6.x/mingw-w64-crt/misc/difftime.c b/mingw-w64-v6.x/mingw-w64-crt/misc/difftime.c
index 5d49f52..e69de29 100644
--- a/mingw-w64-v6.x/mingw-w64-crt/misc/difftime.c
+++ b/mingw-w64-v6.x/mingw-w64-crt/misc/difftime.c
@@ -1,21 +0,0 @@
-#define __CRT__NO_INLINE
-#include <time.h>
-#include <memory.h>
-
-/* FIXME: Relying on _USE_32BIT_TIME_T, which is a user-macro,
-during CRT compilation is plainly broken.  Need an appropriate
-implementation to provide users the ability of compiling the
-CRT only with 32-bit time_t behavior. */
-
-#ifndef _USE_32BIT_TIME_T
-double __cdecl difftime(time_t _Time1,time_t _Time2)
-{
-  return _difftime64(_Time1,_Time2);
-}
-#else
-double __cdecl difftime(time_t _Time1,time_t _Time2)
-{
-  return _difftime32(_Time1,_Time2);
-}
-#endif
-