fix abba deadlock in Sequence.flushInternal

after my previous commit which removed the use of the system session. Since we are taking the meta lock here, we don't need any extra synchronization

h2/src/main/org/h2/schema/Sequence.java

@@ -32,7 +32,6 @@ public class Sequence extends SchemaObjectBase {
     private long maxValue;
     private boolean cycle;
     private boolean belongsToTable;
-    private final Object flushSync = new Object();
     private boolean writeWithMargin;
 
     /**
@@ -306,7 +305,6 @@ public class Sequence extends SchemaObjectBase {
     }
 
     private void flushInternal(Session session) {
-        synchronized (flushSync) {
         final boolean metaWasLocked = database.lockMeta(session);
         // just for this case, use the value with the margin
         try {
@@ -319,7 +317,6 @@ public class Sequence extends SchemaObjectBase {
             database.unlockMeta(session);
         }
     }
-    }
 
     /**
      * Flush the current value to disk and close this object.