Fix and enable unused-import lint

.pylintrc

@@ -17,7 +17,6 @@
 #TODO: Enable cyclic-import
 #TODO: Enable unused-variable
 #TODO: Enable redefined-outer-name
-#TODO: Enable unused-import
 #TODO: Enable too-many-instance-attributes
 #TODO: Enable broad-except
 #TODO: Enable too-many-locals
@@ -30,4 +29,4 @@
 #TODO: Enable too-many-nested-blocks
 #TODO: Enable super-init-not-called
 
-disable=missing-docstring,too-few-public-methods,too-many-arguments,no-init,duplicate-code,invalid-name,suppressed-message,locally-disabled,protected-access,no-name-in-module,unused-argument,fixme,wrong-import-order,no-value-for-parameter,cyclic-import,unused-variable,redefined-outer-name,unused-import,too-many-instance-attributes,broad-except,too-many-locals,too-many-lines,redefined-variable-type,next-method-called,import-error,useless-else-on-loop,too-many-return-statements,too-many-nested-blocks,super-init-not-called
+disable=missing-docstring,too-few-public-methods,too-many-arguments,no-init,duplicate-code,invalid-name,suppressed-message,locally-disabled,protected-access,no-name-in-module,unused-argument,fixme,wrong-import-order,no-value-for-parameter,cyclic-import,unused-variable,redefined-outer-name,too-many-instance-attributes,broad-except,too-many-locals,too-many-lines,redefined-variable-type,next-method-called,import-error,useless-else-on-loop,too-many-return-statements,too-many-nested-blocks,super-init-not-called

src/python/grpcio/grpc/beta/_client_adaptations.py

@@ -30,7 +30,6 @@
 
 import grpc
 from grpc import _common
-from grpc._cython import cygrpc
 from grpc.beta import interfaces
 from grpc.framework.common import cardinality
 from grpc.framework.foundation import future

src/python/grpcio/grpc/beta/implementations.py

@@ -29,16 +29,15 @@
 """Entry points into the Beta API of gRPC Python."""
 
 # threading is referenced from specification in this module.
-import abc
-import enum
 import threading  # pylint: disable=unused-import
 
-# cardinality and face are referenced from specification in this module.
+# interfaces, cardinality, and face are referenced from specification in this
+# module.
 import grpc
 from grpc import _auth
 from grpc.beta import _client_adaptations
 from grpc.beta import _server_adaptations
-from grpc.beta import interfaces
+from grpc.beta import interfaces  # pylint: disable=unused-import
 from grpc.framework.common import cardinality  # pylint: disable=unused-import
 from grpc.framework.interfaces.face import face  # pylint: disable=unused-import