drm: renesas: shmobile: Remove custom plane destroy callback

There is no need to call drm_plane_force_disable() from the plane's
.destroy() callback, as the plane should have been disabled already
before.  See also commit 3c858a33 ("drm/plane_helper: don't
disable plane in destroy function") for the generic plane helper case.

After removing this call, shmob_drm_plane_destroy() becomes a simple
wrapper around shmob_drm_plane_destroy(), hence replace it by the
latter.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/8d7a3f250612085fdf4e06d377843e8f874b22d9.1694767209.git.geert+renesas@glider.be