archivers/p7zip: Drop maintainership.

I have not had time to properly maintain this port in ages, and do not
think this will change in the foreseeable future.

Someone needs to push bug 228239 beyond the finishing line (there is
some confusion about whether the patch in Debian is enough or not), and
hopefully investigate whether a p7zip fork should be adopted, or
persuade the 7-Zip maintainer to publish the code for their Linux
version (see comment #11 in the PR I mentioned).

PR: 228239