Clean up `BP_Signup` class.

* Fix issue with sign-up meta not being populated correctly.
* Remove stray semi-colons.
* Code formatting clean-up.
* See #5374.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8184 cdf35c40-ae34-48e0-9cc9-0c9da1808c22