gnu: Remove icu4c-68.

* gnu/packages/icu4c.scm (icu4c-68): Delete variable.

Only 0ad depended on it, and the dependency has been upgraded in the
previous commit.

Change-Id: If9d5741f25329ea0026401b37b30a7ab7731e3d9

gnu/packages/icu4c.scm

@@ -177,22 +177,6 @@ C/C++ part.")
                (base32
                 "0icps0avkwy5df3wwc5kybxcg63hcgk4phdh9g244g0xrmx7pfjc"))))))
 
-(define-public icu4c-68
-  (package
-    (inherit icu4c)
-    (version "68.2")
-    (source (origin
-              (method url-fetch)
-              (uri (string-append
-                    "https://github.com/unicode-org/icu/releases/download/release-"
-                    (string-map (lambda (x) (if (char=? x #\.) #\- x)) version)
-                    "/icu4c-"
-                    (string-map (lambda (x) (if (char=? x #\.) #\_ x)) version)
-                    "-src.tgz"))
-              (sha256
-               (base32
-                "09fng7a80xj8d5r1cgbgq8r47dsw5jsr6si9p2cj2ylhwgg974f7"))))))
-
 (define-public icu4c-build-root
   (package
     (inherit icu4c)