Revert "tests/xstatx.c: fix stat syscall tests on mips64"

Previous commit effectively changed types of st_atime, st_ctime, and
st_mtime members of struct stat and struct stat64 to signed integers,
making the mips64 workaround obsolete.

This reverts commit 3fb84bfc79949c145197c61fbf04ce18464e9112.

* tests/xstatx.c (create_sample) [__mips64]: Remove.

tests/xstatx.c

@@ -188,12 +188,7 @@ static int
 create_sample(const char *fname, const libc_off_t size)
 {
 	static const struct timespec ts[] = {
-#ifdef __mips64
-		// On mips64, struct stat cannot handle negative timestamps
-		{10841, 246}, {10843, 135}
-#else
 		{-10843, 135}, {-10841, 246}
-#endif
 	};
 
 	(void) close(0);