chore(python): remove configure_previous_major_version_branches (#2172)

chore(python): remove configure_previous_major_version_branches from python post processor

Source-Link: https://github.com/googleapis/synthtool/commit/6702a344265de050bceaff45d62358bb0023ba7d
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>