libc/*/SYS.h: remove SYCALL macro

This has not been a univerally available interface since it was removed
from amd64 by commit efbef97de9805dec95db09953426f0b21bb2964f in 2004.
I removed the last consumers in 2016 when I replaced pipe(2) with
pipe2(2) in commit b60998c6331e2be21f1fc29fa6c82d94a1f72ef0.

Reviewed by: imp, jhibbits
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42996