Revert "asciidoc: properly differentiate the "-full" variant in meta.description"

I changed my mind about this, I no longer think that meta.description
should differ depending on the package configuration. A package
configuration can be very fine grained, and I don't think it's
possible/preferable to describe this in meta.description.

This reverts commit 9033821c34d5cd69d814d73413da7a2e5f719d3f.
asciidoc: add enableExtraPlugins expression argument

'enableExtraPlugins = true' enables all the extra plugins (filters and
backends) that are available in the expression. This simplifies the
configuration for users who want to have the fullest possible asciidoc
configuration (not needing to enable each individual plugin).