livecheck: remove unused URI require

`URI#parse` was originally added in #9074 and replaced with
`Addressable::URI#parse` in #13306 but `require "uri"` wasn't removed
at the time. livecheck doesn't use `URI` now, so this removes the
unused `require`.

Library/Homebrew/livecheck/livecheck.rb

@@ -7,7 +7,6 @@ require "livecheck/livecheck_version"
 require "livecheck/skip_conditions"
 require "livecheck/strategy"
 require "addressable"
-require "uri"
 
 module Homebrew
   # The {Livecheck} module consists of methods used by the `brew livecheck`