Remove bigquery example already migrated to AIP-47

[chenglongyan]

example_bigquery_to_gcs.py has been moved to

airflow/tests/system/providers/google/bigquery/example_bigquery_to_gcs.py

Lines 36 to 42 in 7f986b3

	DAG_ID = "bigquery_to_gcs"
	DATASET_NAME = f"dataset_{DAG_ID}_{ENV_ID}"
	BUCKET_NAME = f"bucket_{DAG_ID}_{ENV_ID}"
	PROJECT_ID = os.environ.get("SYSTEM_TESTS_GCP_PROJECT")
	BUCKET_FILE = "test.csv"
	TABLE = "test"

related: #22430, #22447, #22311

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragement file, named {pr_number}.significant.rst, in newsfragments.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.