community/whois: remove mkpasswd-doc subpackage

The general rule of thumb is: Only one -doc subpackage per package. Not following this rule causes some confusion, for instance I was wondering why mkpasswd-doc was installed on my system even though mkpasswd wasn't.

Enforcing this rules requires moving the install-mkpasswd Makefile target to package() as documentation files (e.g. man pages) installed to a $subpkgdir are not considered by the default_doc split function.