Remove (near-)duplicate definitions of size_t and ssize_t.

The near duplicates upset fussier compilers that insist that
typedefs be exactly the same, but the fix isn't to make all
copies identical...

Change-Id: Icfdace41726f36ec33c9ae919dbb5a54d3529cc9

libc/include/stdint.h

@@ -226,12 +226,6 @@ typedef int64_t  intmax_t;
 #  define UINTMAX_C(c)	UINT64_C(c)
 #endif
 
-/* size_t is defined by the GCC-specific <stddef.h> */
-#ifndef _SSIZE_T_DEFINED_
-#define _SSIZE_T_DEFINED_
-typedef long int  ssize_t;
-#endif
-
 #define _BITSIZE 32
 
 /* Keep the kernel from trying to define these types... */