drm/imx: Drop unused imx-ipuv3-crtc.o build

Since

commit 3d1df96a ("drm/imx: merge imx-drm-core and ipuv3-crtc in one module")

the former contents of imx-ipuv3-crtc.o are built via imxdrm-objs. So
there's no need to keep an extra entry with a non existing config value
(CONFIG_DRM_IMX_IPUV3).

Fixes: 3d1df96a ("drm/imx: merge imx-drm-core and ipuv3-crtc in one module")
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>