feat: remove allowFallback, lkets see what breaks

Closes #3420198

core/config/schema/core.data_types.schema.yml

@@ -777,9 +777,6 @@ field.field_settings.entity_reference:
         PluginExists:
           manager: plugin.manager.entity_reference_selection
           interface: 'Drupal\Core\Entity\EntityReferenceSelection\SelectionInterface'
-          # @todo Remove this line and explicitly require valid entity reference
-          # selection plugin IDs in https://drupal.org/i/3420198.
-          allowFallback: true
     handler_settings:
       type: entity_reference_selection.[%parent.handler]
       label: 'Entity reference selection plugin settings'