Remove unecessary test dependency on plexus-container-default

No longer required due to Doxia 2.0.0 upgrade and inclusion triggers Maven warning:

```
Plugin [INTERNAL, EXTERNAL] validation issues were detected in following plugin(s)

 * org.apache.maven.plugins:maven-checkstyle-plugin:3.3.1
  Plugin EXTERNAL issue(s):
   * Plugin depends on plexus-container-default, which is EOL
```

pom.xml

@@ -207,12 +207,6 @@ under the License.
     </dependency>
 
     <!-- test -->
-    <dependency>
-      <groupId>org.codehaus.plexus</groupId>
-      <artifactId>plexus-container-default</artifactId>
-      <version>2.1.1</version>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>