chore: remove unused imports (#532)

- [ ] Regenerate this pull request now.

PiperOrigin-RevId: 438616112

Source-Link: googleapis/googleapis@33dd46b

Source-Link: googleapis/googleapis-gen@93fe738
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiOTNmZTczOGQyZjQxYzhlZmUzMzIzNWM5NmVjMzJmMmJhNzViZjIxZiJ9

Author: gcf-owl-bot[bot]

packages/google-cloud-bigquery-datatransfer/protos/google/cloud/bigquery/datatransfer/v1/transfer.proto

@@ -18,7 +18,6 @@ package google.cloud.bigquery.datatransfer.v1;
 
 import "google/api/field_behavior.proto";
 import "google/api/resource.proto";
-import "google/protobuf/duration.proto";
 import "google/protobuf/struct.proto";
 import "google/protobuf/timestamp.proto";
 import "google/rpc/status.proto";