Issue #2843934 by mpdonadio, Gábor Hojtsy: datetime.schema.yml does not use...

Issue #2843934 by mpdonadio, Gábor Hojtsy: datetime.schema.yml does not use the "built-in" date_format type

core/modules/datetime/config/schema/datetime.schema.yml

@@ -47,10 +47,8 @@ field.formatter.settings.datetime_custom:
   label: 'Datetime custom display format settings'
   mapping:
     date_format:
-      type: string
+      type: date_format
       label: 'Date/time format'
-      translatable: true
-      translation context: 'PHP date format'
 
 field.formatter.settings.datetime_time_ago:
   type: mapping