Eliminate use of `extract()` in `validate_user_signup()`. 

`$orig_username` does not need to be pulled from `validate_user_form()` as it is not used in this function.

See #22400.

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git-svn-id: http://core.svn.wordpress.org/trunk@28273 1a063a9b-81f0-0310-95a4-ce76da25c4cd