Remove clock_settime from macOS

Doesn't work on iOS.

Author: ekmartin

src/unix/bsd/apple/mod.rs

@@ -1984,7 +1984,6 @@ extern {
     pub fn aio_cancel(fd: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
     pub fn clock_getres(clk_id: ::clockid_t, tp: *mut ::timespec) -> ::c_int;
     pub fn clock_gettime(clk_id: ::clockid_t, tp: *mut ::timespec) -> ::c_int;
-    pub fn clock_settime(clk_id: ::clockid_t, tp: *const ::timespec) -> ::c_int;
     pub fn lio_listio(mode: ::c_int, aiocb_list: *const *mut aiocb,
                       nitems: ::c_int, sevp: *mut sigevent) -> ::c_int;