chore(python): remove configure_previous_major_version_branches

Source-Link: googleapis/synthtool@6702a34 Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:fbbc8db67afd8b7d71bf694c5081a32da0c528eba166fbcffb3b6e56ddf907d5

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-python:latest
- digest: sha256:5581906b957284864632cde4e9c51d1cc66b0094990b27e689132fe5cd036046
-# created: 2025-02-21T19:32:52.01306189Z
+ digest: sha256:fbbc8db67afd8b7d71bf694c5081a32da0c528eba166fbcffb3b6e56ddf907d5
+# created: 2025-10-30T00:16:55.473963098Z