For sites with many entity types and bundles `og_get_all_group_entity()` becomes unnecessarily slow.
The function uses `og_is_group_type()` to determine whether a bundle is a group type. This function checks the bundles of field `OG_GROUP_FIELD`.
With this change `og_get_all_group_entity()` reads the bundles of `OG_GROUP_FIELD` directly, which is much faster.