Remove *.pm format stuff until it's working

The *.pm (Acme::Cow) Perl module based cowfile format never got fully implemented, and it's not working. But there are still things for it in the code and installations:

*.pm cowfiles present in the default cow set
*.pm files recognized as cowfiles by cowsay -l

Packagers like Debian and I think Arch are patching the installation to get rid of them, for a better user experience.

package() {
 cd ${pkgname}
 make DESTDIR="${pkgdir}" install prefix=/usr
 # missing Acme::Cow module
 rm "${pkgdir}"/usr/share/cowsay/cows/*.pm
}

Let's remove them, so they're not part of the default installation, and the in-repo dev behavior more closely reflects what users will see in an installed cowsay.

I think the special "*.pm format cows are not implemented yet" message when you try to invoke a .pm-format cow is fine to keep.

cowsay-org / cowsay

Remove *.pm format stuff until it's working #54