Make network_admin_url() return admin_url() when not multisite like its home...

Make network_admin_url() return admin_url() when not multisite like its home and site counterparts. Kill the now redundant maybe_network_admin_url() function. fixes #15840.

git-svn-id: https://core.svn.wordpress.org/trunk@16994 1a063a9b-81f0-0310-95a4-ce76da25c4cd