Remove Servlet API 3.0 dependency references.

As detailed in the comments to [GRAILS-11466](https://jira.grails.org/browse/GRAILS-11466) and [GRAILS-11450](https://jira.grails.org/browse/GRAILS-11450) the Servlet API 3.0 dependency no longer exists - but it's in the 2.4.1 docs now.

It would be great if the docs could be updated between grails-core releases, too.