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>