Issue #2197873 by Sutharsan: Clean-up system.schema.yml to remove deprecated system.fast_404.

core/modules/system/config/schema/system.schema.yml

@@ -147,23 +147,6 @@ system.date_format.*:
       type: string
       label: 'Default language'
 
-system.fast_404:
-  type: mapping
-  label: 'Fast 404 settings'
-  mapping:
-    enabled:
-      type: boolean
-      label: 'Enabled'
-    paths:
-      type: string
-      label: 'Paths'
-    exclude_paths:
-      type: string
-      label: 'Exclude paths'
-    html:
-      type: string
-      label: 'HTML'
-
 system.filter:
   type: mapping
   label: 'Filter settings'