Revert "Add qsort_r() implementation to the C library."

This reverts commit 754c178ae5.

Turns out we don't need it afterall (needed a stable sort anyways).
So, we'll make that change in the dev branch instead.

libc/include/stdlib.h

@@ -101,7 +101,6 @@ extern void * bsearch(const void *key, const void *base0,
 	int (*compar)(const void *, const void *));
 
 extern void qsort(void *, size_t, size_t, int (*)(const void *, const void *));
-extern void qsort_r(void *a, size_t n, size_t es, void *thunk, int (*)(void *, const void *, const void *));
 
 extern long jrand48(unsigned short *);
 extern long mrand48(void);