MDL-74802 core: Remove tooltip on the notification close button

Reverting the addition of a Boostrap tooltip on the alert notification's
close button. It introduced a bug where the tooltip remains after
dismissing the notification alert. We can manually toggle the state of
the tooltip via JS, but I don't think it's worth the effort. The tooltip
is meant as an added bonus for sighted users to see what the close
button is about.

lib/templates/notification_base.mustache

@@ -40,8 +40,7 @@
         }}{{# announce }} role="alert" data-aria-autofocus="true"{{/ announce }}>
     {{{ message }}}
     {{# closebutton }}{{!
-    }}<button type="button" class="close" data-dismiss="alert" title="{{#str}}dismissnotification, core{{/str}}"{{!
-            }} data-toggle="tooltip">
+    }}<button type="button" class="close" data-dismiss="alert">
         <span aria-hidden="true">&times;</span>
         <span class="sr-only">{{#str}}dismissnotification, core{{/str}}</span>
     </button>{{!