Removed redundant synchronization lock on network switch function

Was useless as done in an already synchronized block, and the lock
object was assigned a new value in that same block, and nowhere else a
lock is requested on that same object.

source/net/yacy/search/Switchboard.java

@@ -1471,9 +1471,7 @@ public final class Switchboard extends serverSwitch {
             this.crawlStacker.announceClose();
             this.crawlStacker.close();
             
-            synchronized ( this.index ) {
-                this.index.close();
-            }
+            this.index.close();
             this.webStructure.close();
 
             this.log.info("SWITCH NETWORK: START UP OF NEW INDEX DATABASE...");