Proactively remove /usr/lib/kgdb file that became a directory

This was already handled in ObsoleteFiles.inc (see the 20201215 entry),
but some people never run "make delete-old", or want to upgrade directly
from a revision that still had the file to the most recent revision.

They would then encounter a failure during installworld, similar to:

install: /usr/libexec/kgdb exists but is not a directory

Therefore, clean it up in the distrib-cleanup phase, similar to the
earlier instances of libc++ header files that became a directory.

MFC after: 3 days