Remove docker-commons from sample-plugin dependencies

README advises that the dependencies in the sample plugin should be
maintained as we would any other plugin, with the top level
dependencies declared and the transitive dependencies calculated by
Maven.

Since docker-workflow depends on docker-commons, we don't need to
declare docker-commons in the sample plugin.

sample-plugin/pom.xml

@@ -419,11 +419,6 @@
       <artifactId>dashboard-view</artifactId>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins</groupId>
-      <artifactId>docker-commons</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>docker-java-api</artifactId>