MDL-57585 dashboard: remove side block region

Made the central block region the default region and removed the side
block region on the dashboard to allow the middle block region more
space.

Part of MDL-55611 epic.

theme/boost/config.php

@@ -79,8 +79,8 @@ $THEME->layouts = [
     // My dashboard page.
     'mydashboard' => array(
         'file' => 'columns2.php',
-        'regions' => array('side-pre'),
-        'defaultregion' => 'side-pre',
+        'regions' => array('content'),
+        'defaultregion' => 'content',
         'options' => array('nonavbar' => true, 'langmenu' => true),
     ),
     // My public page.