Remove helper functions for assessing the state of the `wp_signups` table, as they are no longer used in multiple places. Instead, suppress errors and always query for the existence of the table and `signup_id` column before attempting to upgrade or install. See #5563. Props r-a-y. (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8313 cdf35c40-ae34-48e0-9cc9-0c9da1808c22