Remove external plugin support

Jagex has requested I take steps to prevent users from making plugins
which are in violation of the game rules. This limits plugins to the core
plugins.

travis/settings.xml

@@ -265,7 +265,6 @@ under the License.
         <runelite.tomcat.url>http://api.runelite.net/manager/text</runelite.tomcat.url>
         <maven.javadoc.skip>false</maven.javadoc.skip>
         <checkstyle.skip>false</checkstyle.skip>
-        <archetype.test.skip>false</archetype.test.skip>
       </properties>
     </profile>
   </profiles>