Refactor deleteCursor method: Change to private and reset cursorId after deletion

- Changed the visibility of the deleteCursor method to private to encapsulate cursor deletion logic.
- Added logic to reset cursorId to null after successfully deleting the cursor.
- Ensured better encapsulation and state management within the KintoneClient class.

Author: kentoyoshida

src/main/java/org/embulk/input/kintone/KintoneClient.java

@@ -125,11 +125,12 @@ public void createCursor(final PluginTask task, final Schema schema)
         }
     }
 
-    public void deleteCursor()
+    private void deleteCursor()
     {
         if (this.cursorId != null) {
             try {
                 this.recordClient.deleteCursor(this.cursorId);
+                this.cursorId = null;
             }
             catch (KintoneApiRuntimeException e) {
                 this.logger.error(e.toString());