Remove redundant number_of_messages from commitments

Context

Since the introduction of a case for stuck states (denoted by None ) in the dissections, we know that it is possible to refute the claim that a given state is the last one because there are no more messages to read. Hence, a valid commitment defined by the pair (final state_hash + number_of_ticks) completely determines the number of messages consumed by the commitment: it must be the number of messages in the inbox between the level of the parent commitment and the level of the commitment itself.

I (@vch9) take over the mr, so if you want to assign someone to correct the code, please assign me.

Manually testing the MR

CI should stay green.

Checklist

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