MDL-68148 core: remove cellspacing

The cellspacing attribute of <table> is not supported in HTML5.
So the removed line did not have any effect since ages ago.

lib/tablelib.php

@@ -1594,7 +1594,6 @@ class table_sql extends flexible_table {
     function __construct($uniqueid) {
         parent::__construct($uniqueid);
         // some sensible defaults
-        $this->set_attribute('cellspacing', '0');
         $this->set_attribute('class', 'generaltable generalbox');
     }