Clean up duplicate hook notations and adjacency for calls to the `wp_signup_location` filter.

Also adds braces missed in [25535].

See #26869.

Built from https://develop.svn.wordpress.org/trunk@28215


git-svn-id: http://core.svn.wordpress.org/trunk@28045 1a063a9b-81f0-0310-95a4-ce76da25c4cd