Removing duplicate aar jars for Proguard

Just like #426 altered DexMojo to recognise that AAR classes.jar and AAR\libs\*.jar (and APKLIB\libs\*.jar) are already in the dependency tree (thanks to the ClasspathModifiderLifecycleParticipant), this change does the same thing for ProguardMojo.

It essentially removes the duplicate AAR classes.jar from the Proguard input.