Regenerate client list in README (googleapis#6784)

This PR was generated using Autosynth. 🌈


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

```
2020-12-21 04:05:16,450 synthtool [DEBUG] > Executing /home/kbuilder/.cache/synthtool/google-api-java-client-services/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2020-12-21 04:05:17,316 synthtool [DEBUG] > Cloning https://github.com/googleapis/discovery-artifact-manager.git.
DEBUG:synthtool:Cloning https://github.com/googleapis/discovery-artifact-manager.git.
2020-12-21 04:05:17,709 synthtool [DEBUG] > Cleaning output directory.
DEBUG:synthtool:Cleaning output directory.
2020-12-21 04:05:17,712 synthtool [DEBUG] > Installing dependencies.
DEBUG:synthtool:Installing dependencies.
removing unused service versions
deleting v1alpha2 of google-api-services-bigqueryreservation
2020-12-21 04:05:19,794 synthtool [DEBUG] > Wrote metadata to synth.metadata.
DEBUG:synthtool:Wrote metadata to synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/41471250-e4f7-4914-bf4b-b1399c9f6627/targets

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Author: yoshi-automation

README.md

@@ -45,7 +45,7 @@ packages on [Maven Central][maven-search].
 | BigQuery API | [v2](clients/google-api-services-bigquery/v2) |
 | BigQuery Connection API | [v1beta1](clients/google-api-services-bigqueryconnection/v1beta1) |
 | BigQuery Data Transfer API | [v1](clients/google-api-services-bigquerydatatransfer/v1) |
-| BigQuery Reservation API | [v1](clients/google-api-services-bigqueryreservation/v1), [v1alpha2](clients/google-api-services-bigqueryreservation/v1alpha2), [v1beta1](clients/google-api-services-bigqueryreservation/v1beta1) |
+| BigQuery Reservation API | [v1](clients/google-api-services-bigqueryreservation/v1), [v1beta1](clients/google-api-services-bigqueryreservation/v1beta1) |
 | Binary Authorization API | [v1](clients/google-api-services-binaryauthorization/v1), [v1beta1](clients/google-api-services-binaryauthorization/v1beta1) |
 | Blogger API v3 | [v2](clients/google-api-services-blogger/v2), [v3](clients/google-api-services-blogger/v3) |
 | Books API | [v1](clients/google-api-services-books/v1) |