Disallow usage of ? for returnType of functions with MIRROR nullability

[vbarua]

When defining the return type of a function with MIRROR nullability, include a ? after the return type is meaningless, as the nullability is entirely determined based on the nullability of the inputs. For example, given a definition like:

  - name: "foo"
    impls:
      - args:
          - name: x
            value: i8
        nullability: MIRROR
        return: string?

the ? at the end of return: string? is meangingless.

[yongchul]

Sounds good. This is also aligned with the #850 which explicitly states disregard output nullability.