drm/i915: Remove unnecessary gen6_ppgtt_unmap_pages

We are already unmapping them in gen6_ppgtt_free. This function became
redundant since commit 06fda602
("drm/i915: Create page table allocators").

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>