Revert "ExternalResource: declare after() to throw Throwable (#1421)"

Revert commit cebbf5e.
It breaks code that extends ExternalResource,
overrides after() and calls super.after() in after().

Author: panchenko

src/main/java/org/junit/rules/ExternalResource.java

@@ -77,10 +77,8 @@ protected void before() throws Throwable {
 
     /**
      * Override to tear down your specific external resource.
-     *
-     * @throws Throwable if teardown fails
      */
-    protected void after() throws Throwable {
+    protected void after() {
         // do nothing
     }
 }