Don't wipe out the user_id param passed to BP_XProfile_Field::populate()

This fixes a bug introduced in r3369 in an attempt to remove a WP_DEBUG
notice, but inadvertantly prevented plugins from directly calling the
populate() method using the user_id param.

Fixes #4912

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7117 cdf35c40-ae34-48e0-9cc9-0c9da1808c22