Fix for table heading borders in the editor.

scratchpad/editor-blocks.css

@@ -741,6 +741,7 @@
 }
 
 .editor-block-list__block table.wp-block-table th {
+	border: 0;
 	border-bottom: 1px solid #ddd;
 	font-size: 90%;
 	padding: 0.5em;