remove utterly wrong includes from mips64/n32 bits/stat.h

these were overlooked during review. bits headers are not allowed to
pull in additional headers (note: that rule is currently broken in
other places but just for endian.h). string.h has no place here
anyway, and including bits/alltypes.h without defining macros to
request types from it is a nop.

arch/mips64/bits/stat.h

-#include <string.h>
-#include <bits/alltypes.h>
-
 struct stat {
 	dev_t st_dev;
 	int __pad1[3];

arch/mipsn32/bits/stat.h

-#include <string.h>
-#include <bits/alltypes.h>
-
 struct stat {
 	dev_t st_dev;
 	long __pad1[2];