Avoid unnecessary conditional block

[`GeneratedAttribute#column_name`](https://github.com/rails/rails/blob/master/railties/lib/rails/generators/generated_attribute.rb#L113-L115) checks already for `reference?` on the attribute returning the right name to use as symbol for label and field.

See also https://github.com/rails/rails/commit/b017562382e1c43655eec3456d2a78c1d4407952 where this code was introduced.