Regenerate bigquerydatatransfer client (googleapis#7302)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-03-18 03:39:39,143 synthtool [DEBUG] > Executing /home/kbuilder/.cache/synthtool/google-api-java-client-services/synth.py.
On branch autosynth-bigquerydatatransfer
nothing to commit, working tree clean
2021-03-18 03:39:39,564 synthtool [DEBUG] > Cloning https://github.com/googleapis/discovery-artifact-manager.git.
DEBUG:synthtool:Cloning https://github.com/googleapis/discovery-artifact-manager.git.
2021-03-18 03:39:39,937 synthtool [DEBUG] > Cleaning output directory.
DEBUG:synthtool:Cleaning output directory.
2021-03-18 03:39:39,939 synthtool [DEBUG] > Installing dependencies.
DEBUG:synthtool:Installing dependencies.
2021-03-18 03:39:41,742 synthtool [INFO] > Generating bigquerydatatransfer v1.
INFO:synthtool:Generating bigquerydatatransfer v1.
2021-03-18 03:39:41,743 synthtool [INFO] > 	1.31.0
INFO:synthtool:	1.31.0
WARNING:root:object without properties CheckValidCredsRequest: OrderedDict([(u'type', u'object'), (u'properties', OrderedDict()), (u'id', u'CheckValidCredsRequest'), (u'description', u'A request to determine whether the user has valid credentials. This method is used to limit the number of OAuth popups in the user interface. The user id is inferred from the API call context. If the data source has the Google+ authorization type, this method returns false, as it cannot be determined whether the credentials are already valid merely based on the user id.')])
WARNING:root:object without properties Empty: OrderedDict([(u'description', u'A generic empty message that you can re-use to avoid defining duplicated empty messages in your APIs. A typical example is to use it as the request or the response type of an API method. For instance: service Foo { rpc Bar(google.protobuf.Empty) returns (google.protobuf.Empty); } The JSON representation for `Empty` is empty JSON object `{}`.'), (u'type', u'object'), (u'id', u'Empty'), (u'properties', OrderedDict())])
Writing json metadata to clients/google-api-services-bigquerydatatransfer/v1.metadata.json
2021-03-18 03:39:42,791 synthtool [DEBUG] > Wrote metadata to clients/google-api-services-bigquerydatatransfer/synth.metadata.
DEBUG:synthtool:Wrote metadata to clients/google-api-services-bigquerydatatransfer/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/ccbdc648-29d4-476e-a3c4-7cd5d29c250e/targets

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Author: yoshi-automation