chore: simplify fn prune_messages_to_be_saved #1789

Job	Run time
e2e test for 3-nodes-transfer --env test	3m 47s
e2e test for invoice-ops --env test	3m 13s
e2e test for open-use-close-a-channel --env test	3m 14s
e2e test for udt --env test	3m 30s
e2e test for reestablish --env test	3m 17s
e2e test for cross-chain-hub --env test	3m 31s
e2e test for router-pay --env test	4m 28s
e2e test for udt-router-pay --env test	3m 48s
e2e test for watchtower --env test	5m 1s
e2e test for udt --env xudt-test	3m 28s
e2e test for 3-nodes-transfer --env test --env-var HASH_ALGORITHM=sha256	3m 42s
	40m 59s

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