Remove redundant test assertion incompatible with latest credentials plugin (#3942)

Existing assertions already check that the credential usage tracking
is correct.  This assertion adds no real value to the test.

Testing done:

* Confirmed that tests pass with older credentials plugin and with latest
  credentials plugin.

Author: MarkEWaite

src/test/java/hudson/plugins/git/GitSCMTest.java

@@ -262,7 +262,6 @@ void trackCredentials() throws Exception {
 
         JenkinsRule.WebClient wc = r.createWebClient();
         HtmlPage page = wc.goTo("credentials/store/system/domain/_/credentials/" + credential.getId());
-        assertThat("Have usage tracking reported", page.getElementById("usage"), notNullValue());
         assertThat("No fingerprint created until first use", page.getElementById("usage-missing"), notNullValue());
         assertThat("No fingerprint created until first use", page.getElementById("usage-present"), nullValue());