Don't declare sample plugin transitive dependencies (jenkinsci#2470)

Removes declarations of:

* cloudbees-folder
* favorite
* pipeline-groovy-lib
* pipeline-model-definition
* variant
* workflow-basic-steps
* workflow-durable-task-step
* workflow-multibranch

Attempting to follow the guidance from the README that says:

> Avoid adding transitive dependencies to sample-plugin/pom.xml. It
> is supposed to look as much as possible like a real plugin, and a real
> plugin should only declare its direct dependencies and not its transitive
> dependencies.

sample-plugin/pom.xml

@@ -272,11 +272,6 @@
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>io.jenkins.plugins</groupId>
-      <artifactId>pipeline-groovy-lib</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>io.jenkins.plugins</groupId>
       <artifactId>plugin-util-api</artifactId>
@@ -373,11 +368,6 @@
       <artifactId>cloudbees-bitbucket-branch-source</artifactId>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins</groupId>
-      <artifactId>cloudbees-folder</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>cloudbees-folder</artifactId>
@@ -699,11 +689,6 @@
       <artifactId>unique-id</artifactId>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins</groupId>
-      <artifactId>variant</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>view-job-filters</artifactId>
@@ -800,33 +785,18 @@
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins.workflow</groupId>
-      <artifactId>workflow-basic-steps</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins.workflow</groupId>
       <artifactId>workflow-cps</artifactId>
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins.workflow</groupId>
-      <artifactId>workflow-durable-task-step</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins.workflow</groupId>
       <artifactId>workflow-job</artifactId>
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins.workflow</groupId>
-      <artifactId>workflow-multibranch</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins.workflow</groupId>
       <artifactId>workflow-multibranch</artifactId>
@@ -845,22 +815,12 @@
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jenkinsci.plugins</groupId>
-      <artifactId>pipeline-model-definition</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.jenkinsci.plugins</groupId>
       <artifactId>pipeline-model-definition</artifactId>
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.jvnet.hudson.plugins</groupId>
-      <artifactId>favorite</artifactId>
-      <scope>test</scope>
-    </dependency>
   </dependencies>
   <repositories>
     <repository>