remove unused (and invalid) C version of sigsetjmp

originally, the comment in this code was correct and it would likely
work if the compiler generated a tail call to setjmp. however, commit
583e5512 redesigned sigsetjmp and
siglongjmp such that the old C implementation (which was not intended
to be used) is not even conceptually correct. remove it in the
interest of avoiding confusion when porting to new archs.

src/signal/sigsetjmp.c

-#include <setjmp.h>
-#include <signal.h>
-#include "libc.h"
-
-/* !!! This function will not work unless the compiler performs
- * tail call optimization. Machine-specific asm versions should
- * be created instead even though the workaround (tail call)
- * is entirely non-machine-specific... */
-
-int sigsetjmp(sigjmp_buf buf, int save)
-{
-	if ((buf->__fl = save))
-		pthread_sigmask(SIG_SETMASK, 0, (sigset_t *)buf->__ss);
-	return setjmp(buf);
-}
-
-weak_alias(sigsetjmp, __sigsetjmp);