drm/i915/selftests: drop igt_ppgtt_exhaust_huge

We already have tests that exhaustively exercise the most interesting
page-size combinations, along with tests that offer randomisation, and
so we should already be testing objects(local, system) with a varying
mix of page-sizes, which leaves igt_ppgtt_exhaust_huge providing not
much in terms of extra coverage.

Signed-off-by: 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/20200206170340.102613-1-matthew.auld@intel.com