Formally deprecate the BuddyBar.

BuddyPress has been fully compatible with the WordPress toolbar since 1.6. We
have provided backward compatible support for the BuddyBar through the
BP_USE_WP_ADMIN_BAR constant. This changeset retains this support, but
officially deprecates it, with the following changes:

- All UI related to the BuddyBar has been removed from the Dashboard setings
  panels
- BuddyBar-related code has been moved to the deprecated functions folder
- A _doing_it_wrong() notice is now thrown when loading the BuddyBar

Fixes #5390

Props DJPaul for an initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8568 cdf35c40-ae34-48e0-9cc9-0c9da1808c22