Streamline COUNT query in BP_Activity_Activity::get

It's unnecessary to use an ORDER BY clause when counting, and the index hint
was doing more harm than good, especially in cases where a specific ID was
passed.

See #5448

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8071 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
POT, generated from r8071

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8072 cdf35c40-ae34-48e0-9cc9-0c9da1808c22