gnu: ruby: Remove graft for 2.4.2.

* gnu/packages/ruby.scm (ruby): Update to 2.4.2.
[replacement]: Remove field.
(ruby-2.4.2): Remove variable.

gnu/packages/ruby.scm

@@ -49,8 +49,7 @@
 (define-public ruby
   (package
     (name "ruby")
-    (replacement ruby-2.4.2)
-    (version "2.4.0")
+    (version "2.4.2")
     (source
      (origin
        (method url-fetch)
@@ -59,7 +58,7 @@
                            "/ruby-" version ".tar.xz"))
        (sha256
         (base32
-         "141nnsdk2q83c23p5kl404id8gy1ap261gin48rbjj5sbksgx1rs"))
+         "0dgp4ypk3smrsbh2c249n5pl6nqhpd2igq9484dbsh81sf08k2kl"))
        (modules '((guix build utils)))
        (snippet `(begin
                    ;; Remove bundled libffi
@@ -103,26 +102,6 @@ a focus on simplicity and productivity.")
     (home-page "https://ruby-lang.org")
     (license license:ruby)))
 
-(define-public ruby-2.4.2
-  (package
-    (inherit ruby)
-    (name "ruby")
-    (version "2.4.2")
-    (source
-     (origin
-       (method url-fetch)
-       (uri (string-append "http://cache.ruby-lang.org/pub/ruby/"
-                           (version-major+minor version)
-                           "/ruby-" version ".tar.xz"))
-       (sha256
-        (base32
-         "0dgp4ypk3smrsbh2c249n5pl6nqhpd2igq9484dbsh81sf08k2kl"))
-       (modules '((guix build utils)))
-       (snippet `(begin
-                   ;; Remove bundled libffi
-                   (delete-file-recursively "ext/fiddle/libffi-3.2.1")
-                   #t))))))
-
 (define-public ruby-2.3
   (package
     (inherit ruby)