Enable "Build has no changes" assertion in FreestyleJobTest

Assertion was disabled due to user interface change that has been modified
in Jenkins 2.459 to again report when the build has no changes.

https://issues.jenkins.io/browse/JENKINS-73168 fixes it in Jenkins 2.459.

jenkinsci#1548 is the
issue to remove the workaround.

Revert part of "Work around JENKINS-73034 (jenkinsci#1547)"

Tested by running the modified test with JENKINS_WAR set to a copy of the 2.459 Jenkins war file.

This reverts part of commit 4d71d1b.

src/test/java/core/FreestyleJobTest.java

@@ -100,8 +100,7 @@ public void should_visit_build_with_permalink() {
         b.open();
         assertThat("Permalink link is current URL", driver.getCurrentUrl(), is(expectedUrl));
         assertThat("Build number is correct", b.getNumber(), is(1));
-        // TODO JENKINS-73168
-        //assertThat("Build has no changes", driver, hasContent("No changes"));
+        assertThat("Build has no changes", driver, hasContent("No changes"));
         assertThat("Build is success", b.getResult(), is(Build.Result.SUCCESS.name()));
     }