drm/i915: Remove link to missing "Batchbuffer Pools" documentation

The referenced documentation section has been removed. Remove the
link to avoid warning when building the documentation.

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190830085849.12519-1-joonas.lahtinen@linux.intel.com