remove unused (and invalid) C version of sigsetjmp

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
583e55122e767b1586286a0d9c35e2a4027998ab 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.