Remove default values from git plugin example

Default values for submodule configuration generation are
ignored in git plugin 4.6.0.  Changes from the default values
are untested and expected to be unworkable in earlier plugin
versions.

Remove those settings to reduce "noise"

README.md

@@ -123,7 +123,7 @@ This test will print the call stack of the execution :
          exampleJob.load(src/test/jenkins/lib/utils.jenkins)
             utils.run()
          exampleJob.stage(Checkout, groovy.lang.Closure)
-            exampleJob.checkout({$class=GitSCM, branches=[{name=feature_test}], doGenerateSubmoduleConfigurations=false, extensions=[], submoduleCfg=[], userRemoteConfigs=[{credentialsId=gitlab_git_ssh, url=github.com/lesfurets/JenkinsPipelineUnit.git}]})
+            exampleJob.checkout({$class=GitSCM, branches=[{name=feature_test}], extensions=[], userRemoteConfigs=[{credentialsId=gitlab_git_ssh, url=github.com/lesfurets/JenkinsPipelineUnit.git}]})
             utils.currentRevision()
                utils.sh({returnStdout=true, script=git rev-parse HEAD})
          exampleJob.gitlabBuilds({builds=[build, test]}, groovy.lang.Closure)