IoT: remove unused message exports (via synth).

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/iot/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:75bc07ef34a1de9895c18af54dc503ed3b3f3b52e85062e3360a979d2a0741e7
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/iot/artman_cloudiot.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iot-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/iot/v1/resources.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iot-v1/google/cloud/iot_v1/proto/resources.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/iot/v1/device_manager.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iot-v1/google/cloud/iot_v1/proto/device_manager.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iot-v1/google/cloud/iot_v1/proto.
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/iot/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/gapic/transports/device_manager_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/gapic/device_manager_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/proto/device_manager_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/proto/resources_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/tests/system/gapic/v1/test_system_device_manager_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/gapic/device_manager_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/tests/unit/gapic/v1/test_device_manager_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/proto/device_manager_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/iot/google/cloud/iot_v1/proto/resources_pb2.py
All done! ✨ 🍰 ✨
12 files reformatted, 7 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.