Remove git plugin 4.x test code

Git plugin 5.0.0 has been provided since bom 1763.v092b_8980a_f5e.  No
need to include the special case for git plugin 4.x since this code will
never be run with git plugin 4.x.

Moves forward from 01b9885

Author: MarkEWaite

src/test/java/org/jenkinsci/plugins/workflow/libs/LibraryStepTest.java

@@ -85,11 +85,7 @@ public class LibraryStepTest {
             null, null, Collections.<GitSCMExtension>emptyList())));
         s.setChangelog(false);
         r.assertEqualDataBoundBeans(s, stepTester.configRoundTrip(s));
-        if (r.jenkins.get().getPlugin("git").getWrapper().getVersion().startsWith("4.")) {
-            snippetizerTester.assertRoundTrip(s, "library changelog: false, identifier: 'foo@master', retriever: legacySCM([$class: 'GitSCM', branches: [[name: '${library.foo.version}']], extensions: [], userRemoteConfigs: [[url: 'https://nowhere.net/']]])");
-        } else {
-            snippetizerTester.assertRoundTrip(s, "library changelog: false, identifier: 'foo@master', retriever: legacySCM(scmGit(branches: [[name: '${library.foo.version}']], extensions: [], userRemoteConfigs: [[url: 'https://nowhere.net/']]))");
-        }
+        snippetizerTester.assertRoundTrip(s, "library changelog: false, identifier: 'foo@master', retriever: legacySCM(scmGit(branches: [[name: '${library.foo.version}']], extensions: [], userRemoteConfigs: [[url: 'https://nowhere.net/']]))");
     }
 
     @Test public void vars() throws Exception {