Chore: remove old proof-of-concept (#2282)

* chore: removes old proof of concept

* removes old __init__.py

Author: chalmerlowe

google/cloud/bigquery_v2/__init__.py

@@ -24,7 +24,6 @@
 from .services.routine_service import RoutineServiceClient
 from .services.row_access_policy_service import RowAccessPolicyServiceClient
 from .services.table_service import TableServiceClient
-from .services.centralized_service import BigQueryClient
 
 from .types.biglake_config import BigLakeConfiguration
 from .types.clustering import Clustering
@@ -215,7 +214,6 @@
     "BiEngineReason",
     "BiEngineStatistics",
     "BigLakeConfiguration",
-    "BigQueryClient",
     "BigtableColumn",
     "BigtableColumnFamily",
     "BigtableOptions",