westend: `SignedPhase` is a constant (#3646)

In preparation for the merkleized metadata, we need to ensure that
constants are actually constant. If we want to test the unsigned phase
we could for example just disable signed voter. Or we add some extra
mechanism to the pallet to disable the signed phase from time to time.

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Co-authored-by: Ankan <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>