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
#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>

Author: feanil

openedx/core/djangoapps/common_initialization/checks.py

@@ -40,11 +40,4 @@ def validate_marketing_site_setting(app_configs, **kwargs):  # pylint: disable=u
                 id='common.djangoapps.common_initialization.E002',
             )
         )
-    elif not settings.MKTG_URLS.get('ROOT'):
-        errors.append(
-            Error(
-                'There is no ROOT defined in MKTG_URLS.',
-                id='common.djangoapps.common_initialization.E003',
-            )
-        )
     return errors