MAINTAINERS: Remove Jani as drm-misc co-maintainer

I'm juggling too many things, and drm-misc maintenance is one that I
keep dropping on the floor. Admit reality and remove myself as
maintainer. This still leaves us with a nice team of three who are
actually doing the drm-misc work, while I focus on drm-intel.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Relunctantly-Acked-By: Sean Paul <seanpaul@chromium.org>
Acked-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123121308.12818-1-jani.nikula@intel.com