Exclude generated services from the library

There are currently no public API endpoints for the ModelServiceClient,
thus there is no point in generating that code in the first place.

Author: plamut

google/cloud/bigquery_v2/__init__.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-from .services.model_service import ModelServiceClient
+
 from .types.encryption_config import EncryptionConfiguration
 from .types.model import DeleteModelRequest
 from .types.model import GetModelRequest
@@ -41,5 +41,4 @@
     "StandardSqlDataType",
     "StandardSqlField",
     "StandardSqlStructType",
-    "ModelServiceClient",
 )