i965/gen8: Remove dead assertion

i965/gen8: Remove dead assertion

The assertion is inside a condition mandating num_samples > 1 and
therefore the first half of the constraint is always met. The
second half in turn would only be applicable for single sampled
case and moreover it is trying to falsely check against surface
type instead of format.
Subsequent patches will introduce proper support for the lossless
compression and dropping this here makes the patches a little
simpler.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>