Change _statuses_user_load() calls to user_load()

_statuses_user_load() is deprecated (all it does is call user_load()). We needed it in D6 because user_load() had no static cache so the number of times we needed to load the same user was a significant drag on page load times. However user_load() does use a cache in D7 so there is no need for a wrapper function any more.