datalib: get_admins() - Rewrite to avoid get_users_by_capability()
datalib: get_admins() - Rewrite to avoid get_users_by_capability()

get_admins() and get_admin() were counting on
get_users_by_capability() returning a role-assignment id to pick the
"primary" admin account. With the get_users_by_capability() rewrite,
we no longer have an RA id to clearly blame for the capability.

So, rewrite get_admins() based on the known-good SQL used in
is_siteadmin().

MDL-12452

dilawar/moodle