Remove unnecessary constraint on previous_api

The constraint requires the previous_api property to be specified
unless validating nullness. However, there is no reason within the
Soong code why that should be.

The only use of previous_api is as the argument to the metalava
--migrate-nullness option which is only added when previous_api is
specified. So, there is no reason in the Soong code for the
constraint.

Metalava also does not require the --migrate-nullness option to be
specified unless validating nullness. So, there is no reason in
metalava for the constraint.

It is therefore safe to remove the constraint.

Bug: 142113521
Test: m checkbuild
Change-Id: I189071e215e928fdf43a39a03d540732743b7a32