Remove ancient RtlUnwindEx workaround for MinGW64.

Thanks to Kacper Michajłow. #1272

src/lj_err.c

@@ -384,12 +384,6 @@ typedef struct UndocumentedDispatcherContext {
 /* Another wild guess. */
 extern void __DestructExceptionObject(EXCEPTION_RECORD *rec, int nothrow);
 
-#ifdef MINGW_SDK_INIT
-/* Workaround for broken MinGW64 declaration. */
-VOID RtlUnwindEx_FIXED(PVOID,PVOID,PVOID,PVOID,PVOID,PVOID) asm("RtlUnwindEx");
-#define RtlUnwindEx RtlUnwindEx_FIXED
-#endif
-
 #define LJ_MSVC_EXCODE		((DWORD)0xe06d7363)
 #define LJ_GCC_EXCODE		((DWORD)0x20474343)