samples: drivers: display: Remove explicitly enabled CONFIG_DUMMY_DISPLAY

The CONFIG_DUMMY_DISPLAY is now enabled in conjunction with devicetree,
So, no need to be explicitly enabled.
Remove the setting.

Signed-off-by: TOKITA Hiroshi <[email protected]>

Author: soburi

samples/drivers/display/sample.yaml

@@ -14,7 +14,6 @@ tests:
       - native_sim
     extra_args: DTC_OVERLAY_FILE="dummy_dc.overlay"
     extra_configs:
-      - CONFIG_DUMMY_DISPLAY=y
       - CONFIG_SDL_DISPLAY=n
       - CONFIG_TEST=y
     tags: display