Remove "prev" and "next" links when on a BuddyPress page.

WordPress injects these links in the <head> of each WordPress post.
However, BuddyPress doesn't adhere to these links.  These links also take
two DB queries on each page load and are uncached.

Therefore, this commit removes these links.

Fixes #4981.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8511 cdf35c40-ae34-48e0-9cc9-0c9da1808c22