drm/i915/selftests: Remove unused __engines_name()

This function was never used and probably will never be used, so remove
it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190901110431.12393-1-chris@chris-wilson.co.uk