[cfe] Remove _isNullabilityObliviousSubtypeMatch and related code.

This logic is no longer used. The only call sites were due to mutual
recursion.

Change-Id: I794007f9a9d038e4094251554f5915691cc0e85f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388741
Reviewed-by: Chloe Stefantsova <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

Author: stereotype441