feat!: remove the ENABLE_MFE_CONFIG_API toggle

The LMS setting ENABLE_MFE_CONFIG_API (default False) gated two
read-only endpoints used by frontends:

  GET /api/mfe_config/v1
  GET /api/frontend_site_config/v1/

Both return only non-sensitive frontend configuration, and both are
empty unless an operator populates MFE_CONFIG, MFE_CONFIG_OVERRIDES, or
FRONTEND_SITE_CONFIG.  There was no reason to keep them off by default,
so the toggle is gone and both endpoints now respond unconditionally.

The data settings (MFE_CONFIG, MFE_CONFIG_OVERRIDES,
FRONTEND_SITE_CONFIG) are unchanged and remain how operators control
what these endpoints return.  The toggle was LMS-only; there was no CMS
equivalent.

Removes:
- the toggle definition and annotation in lms/envs/common.py
- the `if not settings.ENABLE_MFE_CONFIG_API` guards in
  MFEConfigView.get and FrontendSiteConfigView.get, plus the
  HttpResponseNotFound / NotFound imports they were the only users of
- the ENABLE_MFE_CONFIG_API overrides in lms/envs/test.py and
  lms/envs/mock.yml
- the two "returns 404 when disabled" tests, which no longer describe
  any reachable behaviour

See DEPR ticket: https://github.com/openedx/openedx-platform/issues/38959

BREAKING CHANGE: Operators who set ENABLE_MFE_CONFIG_API in their
LMS_CFG yaml or a private settings module should drop the line; the key
is now ignored.  If you set it to True, deleting it is all that is
needed -- that is the new default.  If you set it to False *and*
populated MFE_CONFIG, MFE_CONFIG_OVERRIDES, or FRONTEND_SITE_CONFIG,
that configuration is now served publicly by the two endpoints above;
review or clear it if it should not be public.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>