[Python] Remove obsolete CI scripts after CI Gate migration #11873

Context

Follow-up to #11522 — Migrate Python checks to CI Gate, which migrates Python checks from Azure DevOps pipelines to GitHub Actions.

In this review discussion, the author confirmed that unnecessary script files in packages/http-client-python could be removed, but deferred the deletion to a separate PR to keep the migration focused.

Scope

• Identify scripts in packages/http-client-python and related legacy pipeline configuration that become unused after the migration.
• Confirm they are not required by active CI workflows, release automation, or local development before deleting them.
• Remove obsolete files and update affected references or documentation.

Acceptance criteria

Obsolete Python CI scripts and configuration are removed, no dangling references remain, and supported CI and local development workflows continue to work.

Further CI performance optimizations mentioned in #11522 are outside this cleanup's scope.