rptest: remove wait() in nodes_decommissioning_test

The second `producer.wait()` call in `verify()` will fail with the new
assert if this `wait()` is left here.

It is also unnecessary for the test as written.

Author: WillemKauf

tests/rptest/tests/nodes_decommissioning_test.py

@@ -490,7 +490,6 @@ def calculate_total_learners_gap() -> int | None:
 
         self.logger.info(f"decommissioning node: {to_decommission_id}", )
         self._decommission(to_decommission_id)
-        self.producer.wait()
 
         def learner_gap_reported(decommissioned_node_id: int):
             total_gap = calculate_total_learners_gap()