You need to be perfectly aware of the consequences of what you want to do: the effective POM will also contain your current settings (content of settings.xml), thereby possibly publicly exposing whatever passwords you have hard-coded in there. A better solution would be just to deploy the parent POM.
However, if you really want to go down that path, you can have the following configuration:
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>2.4</version>
<configuration>
<archive>
<addMavenDescriptor>false</addMavenDescriptor>
</archive>
</configuration>
</plugin>
<plugin>
<artifactId>maven-help-plugin</artifactId>
<version>2.1.1</version>
<executions>
<execution>
<phase>generate-resources</phase>
<goals>
<goal>effective-pom</goal>
</goals>
<configuration>
<output>${project.build.outputDirectory}/META-INF/maven/${project.groupId}/${project.artifactId}/pom.xml</output>
</configuration>
</execution>
</executions>
</plugin>
This tells the maven-jar-plugin not to add the Maven descriptor pom.xml and pom.properties to the jar. Instead, the pom.xml is generated by the maven-help-plugin and its effective-pom goal.
If you want the pom.properties file also, you will need to create it manually with the maven-antrun-plugin.