drm/ttm/tests: Delete unnecessary config option

DRM KUnit helpers are selected automatically when TTM tests are enabled,
so there's no need to do it directly in the .kunitconfig file.
Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1f6f03ac5c6ba7d4b021c2a31638a0ae086486e8.1718192625.git.karolina.stolarek@intel.com

drivers/gpu/drm/ttm/tests/.kunitconfig

 CONFIG_KUNIT=y
 CONFIG_DRM=y
-CONFIG_DRM_KUNIT_TEST_HELPERS=y
 CONFIG_DRM_TTM_KUNIT_TEST=y