[Python] Remove obsolete CI scripts after CI Gate migration

## Context

Follow-up to [#11522 — Migrate Python checks to CI Gate](https://github.com/microsoft/typespec/pull/11522), which migrates Python checks from Azure DevOps pipelines to GitHub Actions.

In [this review discussion](https://github.com/microsoft/typespec/pull/11522#discussion_r3717369976), 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.