Remove pass-by-reference for objects in selected places in BP

Since PHP 5.0, objects have been passed by reference by default. In more recent
versions of PHP, standards notices have been introduced to increasingly
deprecate the practice of passing objects by reference using an ampersand.

This changeset removes several of these strict standard notices, by removing
the explicit pass-by-reference for the objects in question.

See #5108

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7288 cdf35c40-ae34-48e0-9cc9-0c9da1808c22