Minor fix for table headings in the editor.

scratchpad/editor-blocks.css

@@ -610,6 +610,7 @@
 
 .wp-block-freeform.block-library-rich-text__tinymce table td,
 .wp-block-freeform.block-library-rich-text__tinymce table th {
+	border: 0;
 	border-bottom: 1px solid #ddd;
 	font-size: 90%;
 	padding: 0.5em;