Remove metamail dependency, and add a mailcap.5 manpage.

The intention of r411160 was to provide a mailcap manpage, as referenced
in multiple places by the mutt(1) and muttrc(5) manpages.

Although mail/metamail provides a mailcap.4 manpage, metamail is not
particularly relevant as a mailcap provider. The main external filter
that mutt needs is something to render text/html mime parts, but the
last metamail release was in 1994, 22 years ago.

This commit removes that metamail dependency, and adds a mailcap.5
manpage written by Udo Schweigert (mail/mutt's maintainer), based off
metamail's mailcap.4 manpage.

PR: 207727
Submitted by: Udo Schweigert (maintainer)