feat: remove MKTG_URL_LINK_MAP setting

This setting was the fallback URL resolution mechanism used when
ENABLE_MKTG_SITE=False. Now that the flag is gone, only MKTG_URLS is
used for marketing link resolution.

- Remove MKTG_URL_LINK_MAP definition from openedx/envs/common.py
- Remove LMS-specific update block from lms/envs/common.py
- Remove test environment override from lms/envs/test.py
- Remove YAML loading and dict-update from lms and cms production envs
- Remove the dynamic URL registration loop from static_template_view
- Remove MKTG_URL_LINK_MAP.keys() union from the marketing link
  context processor in edxmako/shortcuts.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>