download: Remove unreachable SourceForge mirror.

* guix/download.scm (%mirrors)[sourceforge]: Remove kent.dl.*, which is
  unreachable.

guix/download.scm

@@ -83,8 +83,7 @@
        "http://dfn.dl.sourceforge.net/sourceforge/"
        "http://mesh.dl.sourceforge.net/sourceforge/"
        "http://ovh.dl.sourceforge.net/sourceforge/"
-       "http://osdn.dl.sourceforge.net/sourceforge/"
-       "http://kent.dl.sourceforge.net/sourceforge/")
+       "http://osdn.dl.sourceforge.net/sourceforge/")
       (kernel.org
        "http://www.all.kernel.org/pub/"
        "http://ramses.wh2.tu-dresden.de/pub/mirrors/kernel.org/"