Don't prime activitymeta cache in BP_Activity_Activity::construct()

bp_activity_get_meta() now uses the core API functions. get_metadata() works by
priming the cache with *all* object metadata when requesting any piece of
metadata for that object. This means that, instead of pre-fetching activitymeta
when instantiating BP_Activity_Activity, we wait until it happens in
get_metadata(). At best, this can help us to avoid pre-fetching a meta cache
that will not be used.

See #5398

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7901 cdf35c40-ae34-48e0-9cc9-0c9da1808c22