Remove post form completely from the /active custom view

[Clorith]

Currently if you visit a plugins forums and append /active to the url it will show a list of activity.

If you are in this custom view, the new thread form shows a message saying you can't create new threads at this time.

This is a special view for listing, and not meant to be used as normal forums, so it makes sense to remove the form for creating threads from the view altogether. (of course if someone feels up to making it work instead, by all means, but it seems "wrong" to have such actions available in a helper view)

[coffee2code]

In 4115:

Support Forums: Remove new topic form from the /active custom view.

Fixes #2047.