Remove deprecated signup_build_signup_data() function

The signup_build_signup_data() function was a deprecated hold-over during the conversion to a bunch of theme functions over at #137609: Theme function for signup_build_signup_data and #178441: Custom signup data doesn't include labels in emails, and has no theme functions. Since D6 is a new era, and signup-related modules are probably going to have to port to the new version of signup in some way, now's a good time to remove the code since it's no longer needed internally.