Remove the implicit display feature of .Lk because it was inconsistent

Remove the implicit display feature of .Lk because it was inconsistent
across output devices, counter-intuitive, and resulted in ugly
output for many real-world manual pages.  Always format even long
links in-line.  I already committed a similar change to groff.

OK jmc@, bentley@, and the original author
of the feature, Werner Lemberg <wl@gnu.org>.