Grunt: remove `backpress` svn:ignore external from the `bp-forums` component.

Trunk no longer has an external of the old 1.x version of bbPress. See r8353.

This commit is part of a series that will re-organise BuddyPress trunk to use a Grunt build process. See #5160


git-svn-id: https://buddypress.svn.wordpress.org/trunk@8362 cdf35c40-ae34-48e0-9cc9-0c9da1808c22