Do not use wp_parse_id_list() twice in static group get() methods.

BP_Groups_Group::get_group_extras() already uses the wp_parse_id_list()
function to sanitize the group IDs.

No need to sanitize before the get_group_extras() method call.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7461 cdf35c40-ae34-48e0-9cc9-0c9da1808c22