Fix a false positive in missing_equatable_props

[PiotrRogulski]

missing_equatable_props asked for super.props whenever the superclass exposed a
concrete props getter, even when that getter returns an empty list. Referencing it
there contributes nothing to equality, so the warning was noise and the quick fix
inserted a no-op entry.

The analyzer can't reach a superclass's props body across libraries, so the check
now leans on the rule's own model of props — the list of a class's non-static
fields. A superclass chain that declares no such fields necessarily has an empty
props, so super.props is only required when some ancestor between the superclass
and Equatable declares a field of its own. Being element-based, this behaves the
same whether the parent lives in the same file or another package.

One deliberate false negative follows: a parent with no fields but a hand-written
props => [someGetter] no longer gets super.props suggested. That matches the rule
already never requiring getters in props.

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