drm/doc: Remove solved KMS cleanup task

Remove KMS cleanup task from documentation solved by patchset
https://patchwork.freedesktop.org/series/54310/



Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208195312.aqv7acr3hgion5yz@smtp.gmail.com