libc: rm stale generated files which are no longer syscalls

This is an attempt to help -DNO_CLEAN builds after r302092 (which
removed the pipe libc syscall wrapper) and r318736 (which removed
getdents, lstat, mknod, and stat).

Dependencies cannot cope with certain source tree changes,
particularly with respect to removing source files and replacing
generated files. Handle these cases from _worldtmp in an ad-hoc
fashion.

Reviewed by: bdrewery, cem
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10876