[CT-1640] Remove custom code to poll status for python model jobs on cluster

[ChenyuLInx]

We added

dbt-bigquery/dbt/adapters/bigquery/python_submissions.py

Lines 70 to 76 in 32085f9

	def _wait_operation(self, operation):
	# can't use due to https://github.com/googleapis/python-api-core/issues/458
	# response = operation.result(retry=self.retry)
	# Temp solution to wait for the job to finish
	start = time.time()
	while not operation.done(retry=None) and time.time() - start < self.timeout:
	time.sleep(OPERATION_RETRY_TIME)

due to issue googleapis/python-api-core#458 (comment). Remove this and use method provided by operation directly