mythplugins: please remove mythweb as a dependency

MythTV plugins are intended to be optionally installed on all local
(same host as mythtv-backend) and remote mythtv-frontend
installations.

The mythweb application is not designed to be a mythtv-frontend
package. It is most usually installed on the mythtv-backend host. It
requires apache2/httpd and access to the mysql database for MythTV.

Local and remote mythtv-frontends access mythweb over http in a
broser. It is not something that should be installed on every
mythtv-frontend host and should there not be included as a dependency
on mythplugins.

It is valid to suggest mythweb for mythtv-backend - as is currently the case.

Cheers,
Nick