T15: remove back-compat ruleset

Author: sabernhardt

src/wp-content/themes/twentyfifteen/css/editor-blocks.css

@@ -339,12 +339,6 @@ Description: Used to style blocks in the editor.
 	max-width: 660px;
 }
 
-.editor-post-title__block,
-.editor-default-block-appender,
-.editor-block-list__block {
-	max-width: 690px; /* Account for padding in older WordPress versions. */
-}
-
 /* Link styles */
 
 .edit-post-visual-editor a,