Validate usernames only once in a multisite signup

[arkimedia]

In wpmu_validate_user_signup function there is a double validation for allowed username.

First, the username is passed in to a sanitize_user function and after that tested with preg_match( '/[^a-z0-9]/', $user_name );

Latter test is more strict than the one used in sanitize user function and doesn't allow for example uppercase characters or dots.

I think that the latter test is unnecessary. If it can't be removed, we should add filter for it.

[chriscct7]

Duplicate of #17904.