i965: Drop the old sw fallback for position array being disabled.

i965: Drop the old sw fallback for position array being disabled.

This code has been in the driver since the first commit.  I think it was
trying to stop rendering from happening with a disabled position array.  Core
mesa has since had changes to deal with disabled position arrays correctly.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>