gnu: Remove unrar.

This package is abandoned upstream and contains serious bugs:

http://seclists.org/oss-sec/2017/q3/329
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-14120
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-14121
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-14122

* gnu/packages/compression.scm (unrar): Remove variable.

gnu/packages/compression.scm

@@ -1285,24 +1285,6 @@ (define-public unshield
  archives from InstallShield installers.")
     (license license:expat)))
 
-(define-public unrar
-  (package
-    (name "unrar")
-    (version "0.0.1")
-    (source (origin
-              (method url-fetch)
-              (uri (string-append
-                    "http://download.gna.org/unrar/unrar-" version ".tar.gz"))
-              (sha256
-               (base32
-                "1fgmjaxffj3shyxgy765jhxwz1cq88hk0fih1bsdzyvymyyz6mz7"))))
-    (build-system gnu-build-system)
-    (home-page "http://download.gna.org/unrar")
-    (synopsis "RAR archive extraction tool")
-    (description "Unrar is a simple command-line program to list and extract
-RAR archives.")
-    (license license:gpl2+)))
-
 (define-public zstd
   (package
     (name "zstd")