chore(python): remove configure_previous_major_version_branches

chore(python): remove configure_previous_major_version_branches from python post processor

Source-Link: googleapis/synthtool@6702a34
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:bb3dc6c75c5f3e1a8928474f2dc8d9bbfa7b809045515764b9ff48c070cf2321

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest
-  digest: sha256:bdf89cdfb5b791d382184a7a769862b15c38e94e7d82b268c58d40d8952720f2
-# created: 2025-10-03T19:51:38.870830821Z
+  digest: sha256:bb3dc6c75c5f3e1a8928474f2dc8d9bbfa7b809045515764b9ff48c070cf2321
+# created: 2025-11-03T16:03:37.116278509Z