gnu: Remove ruby-listen-3.0.

This was used by ruby-jekyll-watch, but that's now been upgraded and now uses
the ruby-listen package.

* gnu/packages/ruby.scm (ruby-listen-3.0): Remove this variable.

gnu/packages/ruby.scm

@@ -4499,17 +4499,6 @@ you about the changes.")
     (home-page "https://github.com/guard/listen")
     (license license:expat)))
 
-(define-public ruby-listen-3.0
-  (package
-    (inherit ruby-listen)
-    (version "3.0.8")
-    (source (origin
-              (method url-fetch)
-              (uri (rubygems-uri "listen" version))
-              (sha256
-               (base32
-                "1l0y7hbyfiwpvk172r28hsdqsifq1ls39hsfmzi1vy4ll0smd14i"))))))
-
 (define-public ruby-loofah
   (package
     (name "ruby-loofah")