Simplify checks on Special:UserLogin/signup

I27c488e4008a924117ff6bac2216f148cd5e7abd changed the validation code in
User::getCanonicalName, the # and Title::makeTitleSafe checks now done
there and can be removed here.

Follow-Up: I27c488e4008a924117ff6bac2216f148cd5e7abd
Follow-Up: I88c479cea2bc9d2eab882e0ee8ebcbe2d1dd125e
Follow-Up: I983e3f528491817f9f31f71a92d8d2946ce5941d
Change-Id: I17bf88be6398dc402295264ca9bfd57809c872d7