Remove PacketExtensionFilter(Class) constructor

as it's potential error prone, because most PacketExtension's don't
have a non-argument constructor required for this to work.

Also add some javadoc.

smack-core/src/main/java/org/jivesoftware/smack/filter/PacketExtensionFilter.java

@@ -57,14 +57,15 @@ public class PacketExtensionFilter implements PacketFilter {
         this(null, namespace);
     }
 
+    /**
+     * Creates a new packet extension filter for the given packet extension.
+     *
+     * @param packetExtension
+     */
     public PacketExtensionFilter(PacketExtension packetExtension) {
         this(packetExtension.getElementName(), packetExtension.getNamespace());
     }
 
-    public PacketExtensionFilter(Class<? extends PacketExtension> packetExtensionClass) throws InstantiationException, IllegalAccessException {
-        this(packetExtensionClass.newInstance());
-    }
-
     public boolean accept(Packet packet) {
         return packet.hasExtension(elementName, namespace);
     }