Removed useless assignment variables

I removed useless assignment variables detected with rubocop.

@drbrain Could you review this?
This is obviously outside of the scope of the PR, but I'm thinking that this self-described hack isn't needed anymore. The line of code was introduced in 2011 by 97932d63efbd. `<=>` is [defined](https://github.com/rubygems/rubygems/blob/942dc0b42bfe3c61e44aa29d5b9c2755069be3c1/lib/rubygems/specification.rb#L1228), so `matches.sort` should work here.