drm/ci: remove rebase-merge directory

Gitlab runner can cache git repository, including the unfinished rebase
merge status. New CI job will come as a fresh checkout, however this
will not destroy the unfinished rebase, failing our build script.
Destroy the unfinished rebase state.

Suggested-by: David Heidelberg <david.heidelberg@collabora.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Helen Koike <helen.koike@collabora.com>
Patchwork: https://patchwork.freedesktop.org/patch/570159/
Signed-off-by: Rob Clark <robdclark@chromium.org>