Tasks: pick up fixes to GAPI generator, drop 'Code' enum.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/tasks/artman_cloudtasks_v2beta2.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/tasks-v2beta2.
synthtool > No replacements made in google/cloud/tasks_v2beta2/gapic/cloud_tasks_client.py for pattern (        \* .*
        )([^\s*]), maybe replacement is not longer needed?
synthtool > Replaced '(Google IAM .*?_) ' in google/cloud/tasks_v2beta2/gapic/cloud_tasks_client.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta2/proto/cloudtasks_pb2.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta2/proto/queue_pb2.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta2/proto/task_pb2.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta2/proto/target_pb2.py.
synthtool > Running generator for google/cloud/tasks/artman_cloudtasks_v2beta3.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/tasks-v2beta3.
synthtool > No replacements made in google/cloud/tasks_v2beta3/gapic/cloud_tasks_client.py for pattern (        \* .*
        )([^\s*]), maybe replacement is not longer needed?
synthtool > Replaced '(Google IAM .*?_) ' in google/cloud/tasks_v2beta3/gapic/cloud_tasks_client.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta3/proto/cloudtasks_pb2.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta3/proto/queue_pb2.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta3/proto/task_pb2.py.
synthtool > Replaced '>`__' in google/cloud/tasks_v2beta3/proto/target_pb2.py.
synthtool > No replacements made in google/cloud/tasks_v2beta2/gapic/cloud_tasks_client.py for pattern (Sample filter \\"app_engine_http_target: )\*\\"., maybe replacement is not longer needed?
synthtool > Replaced '(uests in queue.yaml/xml) <\n\\s+' in google/cloud/tasks_v2beta2/proto/queue_pb2.py.
synthtool > Cleaned up 1 temporary directories.

[tseaver]

Includes fixes from these PRs:

• Remove pending deprecation spew from unit tests. gapic-generator#2407
• Python: expose 'channel' as a property of the transport. gapic-generator#2396