Remove dead stack-checking code.

Code under HAVE_STACK_LIMIT is not used.
The definition was commented out in rev 180 of the original
Mercurial repo, and then completely removed in rev 2520,
but the code guarded by it was left in.

src/os_unix.h

@@ -5,7 +5,6 @@ void mch_write(char_u *s, int len);
 int mch_inchar(char_u *buf, int maxlen, long wtime, int tb_change_cnt);
 int mch_char_avail(void);
 void mch_delay(long msec, int ignoreinput);
-int mch_stackcheck(char *p);
 void mch_startjmp(void);
 void mch_endjmp(void);
 void mch_didjmp(void);