drm/i915: Kill the undead i915_gem_batch_pool.c

You have to cut it off at the neck, otherwise it just reappears in the
next merge, like it did in commit 3f866026f0ce ("Merge drm/drm-next
into drm-intel-next-queued")

References: 3f866026f0ce ("Merge drm/drm-next into drm-intel-next-queued")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190822065917.18988-1-chris@chris-wilson.co.uk