chore(python): remove configure_previous_major_version_branches (#975)

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

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-python:latest
-  digest: sha256:543e209e7c1c1ffe720eb4db1a3f045a75099304fb19aa11a47dc717b8aae2a9
-# created: 2025-10-09T14:48:42.914384887Z
+  digest: sha256:fbbc8db67afd8b7d71bf694c5081a32da0c528eba166fbcffb3b6e56ddf907d5
+# created: 2025-10-30T00:16:55.473963098Z