fix: Fix typehint on AsyncDocumentReference.get() (#649)

* fix: AsyncDocumentReference.get() only returns 1 level of coroutine.

* Remove no-longer-used imports.

Co-authored-by: Mariatta Wijaya <Mariatta@users.noreply.github.com>

google/cloud/firestore_v1/async_document.py

@@ -28,7 +28,7 @@
 from google.cloud.firestore_v1 import _helpers
 from google.cloud.firestore_v1.types import write
 from google.protobuf.timestamp_pb2 import Timestamp
-from typing import Any, AsyncGenerator, Coroutine, Iterable, Union
+from typing import AsyncGenerator, Iterable
 
 
 logger = logging.getLogger(__name__)
@@ -329,7 +329,7 @@ async def get(
         transaction=None,
         retry: retries.Retry = gapic_v1.method.DEFAULT,
         timeout: float = None,
-    ) -> Union[DocumentSnapshot, Coroutine[Any, Any, DocumentSnapshot]]:
+    ) -> DocumentSnapshot:
         """Retrieve a snapshot of the current document.
 
         See :meth:`~google.cloud.firestore_v1.base_client.BaseClient.field_path` for