meson: drop HTML4 variants of rst2html

meson: drop HTML4 variants of rst2html

We stopped supporting Ubuntu 16.04 earlier in the year in:

  commit 57e9f3a7e02988432c8c6790cb058b2b5372821b
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Mon May 4 13:06:56 2020 +0100

    gitlab: update list of distros to use latest versions

So there is no reason to keep the HTML4 variants of rst2html around.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>