code: public Bucket removeRetentionPolicy(String bucketName)
        throws StorageException, IllegalArgumentException {
      // [START storage_remove_retention_policy]
      // Instantiate a Google Cloud Storage client
      Storage storage = StorageOptions.getDefaultInstance().getService();
  
      // The name of a bucket, e.g. "my-bucket"
      // String bucketName = "my-bucket";
  
      Bucket bucket = storage.get(bucketName, BucketGetOption.fields(BucketField.RETENTION_POLICY));
      if (bucket.retentionPolicyIsLocked() != null && bucket.retentionPolicyIsLocked()) {
        throw new IllegalArgumentException(
            "Unable to remove retention period as retention policy is locked.");
      }
  
      Bucket bucketWithoutRetentionPolicy =
          bucket.toBuilder().setRetentionPeriod(null).build().update();
  
      System.out.println("Retention period for " + bucketName + " has been removed");
      // [END storage_remove_retention_policy]
      return bucketWithoutRetentionPolicy;
    }
docs: Example of removing a retention policy on a bucket