[kde-base/kwin] Drop blocker on kde-base/kwin:4

kwin:5 is now co-installable with kwin:4. See also:
http://quickgit.kde.org/?p=kwin.git&a=commit&h=52653aaede30d5e2b6cbefa58be53c1742f7a315

Package-Manager: portage-2.2.10