gitweb/README: remove reference to git.kernel.org

git.kernel.org no longer uses gitweb but has switched to cgit.

Info about this can be found on: https://www.kernel.org/pelican.html
or simply by looking at http://git.kernel.org . This is change since
2013-03-01.
Signed-off-by: NFredrik Gustafsson <iveqy@iveqy.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitweb/README

 GIT web Interface
 =================
 
-The one working on:
-  http://git.kernel.org/
-
 From the git version 1.4.0 gitweb is bundled with git.