chore(python): remove configure_previous_major_version_branches (#2675)

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

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

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:da8a4a745d5eb96f07fa99a550afe49ac80dcd9b93a8d914e4f1f6f1cda653ff
-# created: 2025-10-14T15:58:07.94636513Z
+  digest: sha256:bb3dc6c75c5f3e1a8928474f2dc8d9bbfa7b809045515764b9ff48c070cf2321
+# created: 2025-11-03T16:03:37.116278509Z