Remove never implemented sbrk and sstk syscalls

Both system calls were stubs returning EOPNOTSUPP and libc did not
provide _ or __sys_ prefixed symbols. The actual implementation of
sbrk(2) is on top of the undocumented break(2) system call.

Technically this is a change in ABI, but no non-contrived program ever
called these syscalls.

Reviewed by: kib, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42872