fix assertion exception in toString of MultiProtocolURL

toString of AnchorURL and MultiProtocolURL are identical code
(no need to override or to protect call to parent)

as reported in https://github.com/yacy/yacy_search_server/issues/43

source/net/yacy/cora/document/id/AnchorURL.java

@@ -168,9 +168,4 @@ public class AnchorURL extends DigestURL {
                 ">" + this.textBody + "</a>";
     }
     
-    @Override
-    public String toString() {
-        return this.toNormalform(false);
-    }
-    
 }

source/net/yacy/cora/document/id/MultiProtocolURL.java

@@ -967,7 +967,6 @@ public class MultiProtocolURL implements Serializable, Comparable<MultiProtocolU
 
     @Override
     public String toString() {
-        assert false; // this shall not be used to avoid confusion with AnchorURL.toString
         return toNormalform(false);
     }