rm stale ptrace dependencies after r305012

This is similar to r318912, except that ptrace.[sS] was previously a
file in the source tree, not a generated assembly wrapper.

Check for the existence of ptrace.[sS] in the .depend file to determine
if we have to clean it up. This is a bit hackish and will not be left
in place indefinitely, but provides a useful example case when
investigating a better solution in bmake.

Reviewed by: bdrewery
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10930