fix: remove E003 system check for MKTG_URLS ROOT key

The E003 check required MKTG_URLS['ROOT'] to be defined, but this key
is optional. The index() view in branding/views.py already handles the
case where ROOT is absent:

    root_url = marketing_urls.get("ROOT")
    if root_url and root_url != getattr(settings, "LMS_ROOT_URL", None):
        return redirect(root_url)

Previously, E003 was only triggered when ENABLE_MKTG_SITE=True (the
check was guarded by that flag). After removing ENABLE_MKTG_SITE in
https://github.com/openedx/edx-platform/pull/38720, the check runs
unconditionally and fires in any environment where MKTG_URLS has no
ROOT key (including test and CMS environments). Since ROOT is genuinely
optional, the check is incorrect and should not be enforced.

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