Proto,tx_rollup: Remove redundant size checks for messages

Context

This is a small MR intended to clean up a bit the interface of the transaction rollup inboxes. This is a compromise, in the wait of a final solution in !4405 (closed).

The size of a batch is already checked in the precheck function.

Manually testing the MR

The checks are covered by tests; if the removal was erroneous, the tests would fail. Therefore, a green CI is all that we need.

Checklist

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