Remove setup_request_id fallback in generated packages when google-api-core >= 2.26.0

## Overview

During the GAPIC request-id deduplication effort (centralizing request ID generation into `google.api_core.gapic_v1.requests.setup_request_id`), a backward-compatibility fallback module (`_compat.py`) was introduced in generated client packages to maintain compatibility with older versions of `google-api-core`.

Once `google-api-core >= 2.26.0` (or the version containing `setup_request_id`) becomes the minimum required version across all generated client packages, this fallback can be removed.

## Tasks

- [ ] Remove `_compat.py.j2` template from `gapic-generator-python`.
- [ ] Update `client.py.j2` and `async_client.py.j2` to import `setup_request_id` directly from `google.api_core.gapic_v1.requests`.
- [ ] Remove `_compat.py` files from `tests/integration/goldens/`.