[Bugfix] Don't clear the buffer before flushing when closing for flink-1.13 (StarRocks#264)

Signed-off-by: PengFei Li <[email protected]>

Author: banmoy

src/main/java/com/starrocks/connector/flink/manager/StarRocksSinkManager.java

@@ -267,7 +267,6 @@ public synchronized void close() {
         if (!closed) {
             closed = true;
             LOG.info("StarRocks Sink is about to close, loadMetrics: {}.", metricsToString());
-            this.bufferMap.clear();
             if (scheduledFuture != null) {
                 scheduledFuture.cancel(false);
                 scheduler.shutdown();