Revert NoWarn for NU1903

[ellahathaway]

In dotnet/sdk#44028, we added a no warn as a temporary workaround for #4660.

We should revert this nowarn once dotnet/sdk#43993 has been merged.

When doing this work, we should also consider whether the additional infra that was added to support this nowarn is valuable and should be kept.

[ellahathaway]

Note - this change is now in main. We won't be able to remove the no warn in main until we rebootstrap there.

[ellahathaway]

Closing in favor of #4667