Remove extra create topic links

[Clorith]

r7450 added a new Create Topic link on single forum views.

This patch removes the extra link button at the bottom of the page, as it is next to the new topic form. It also removes it from the sidebar, as it's roughly at the same "eye level" as the new button link. On mobile devices the sidebar is pushed down below the list and the create topic link in the sidebar serves little purpose in those scenarios as well.

[obenland]

In 7489:

Support: Remove duplicate links to create topics

Props Clorith.
See [7450].
Fixes #3722.