try to fix maven build error

source/net/yacy/cora/protocol/http/auth/YaCyDigestSchemeFactory.java

@@ -23,8 +23,6 @@ package net.yacy.cora.protocol.http.auth;
 
 import java.nio.charset.Charset;
 
-import org.apache.http.annotation.Contract;
-import org.apache.http.annotation.ThreadingBehavior;
 import org.apache.http.auth.AuthScheme;
 import org.apache.http.auth.AuthSchemeProvider;
 import org.apache.http.protocol.HttpContext;
@@ -34,7 +32,6 @@ import org.apache.http.protocol.HttpContext;
  * {@link YaCyDigestScheme} instances that support use of YaCy encoded password
  * instead of clear-text password.
  */
-@Contract(threading = ThreadingBehavior.IMMUTABLE)
 public class YaCyDigestSchemeFactory implements AuthSchemeProvider {
 
     private final Charset charset;