Start deleting redundant features that are already covered

Start deleting redundant features that are already covered
by mandoc(1) -Tlint, as suggested by the upstream author
and maintainer Thomas Klausner <wiz @ NetBSD>.
The ultimate goal is to get rid of mdoclint and integrate
all its functionality into mandoc, but that will still take
some time, so simplify mdoclint for the time being.
jmc@ "go ahead", OK wiz@, also committed upstream

This first patch removes the AUTHORS check that mandoc does, too.