[C3] Build warnings from duplicate javacc-maven-plugin definition

[Hugh Sparks]

[Cocoon trunk, SVN 1131615]
There are two identical plugin definitions in parent/pom.xml:
...
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>javacc-maven-plugin</artifactId>
         <version>2.6</version>
       </plugin>
...
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>javacc-maven-plugin</artifactId>
         <version>2.6</version>
       </plugin>
...

Maven complains during the build:

   [WARNING] Some problems were encountered while building the effective
model for
       org.apache.cocoon.parent:cocoon-parent:pom:3.0.0-beta-1-SNAPSHOT
   [WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)'
       must be unique but found duplicate declaration of plugin
       org.codehaus.mojo:javacc-maven-plugin @ line 713, column 17
   [WARNING]
   [WARNING] It is highly recommended to fix these problems because they
       threaten the stability of your build.
   [WARNING]
   [WARNING] For this reason, future Maven versions might no longer support
       building such malformed projects.

-Hugh Sparks