refactor: Remove shutdown watchdog method and related test from SparkSQLEngine and ThreadDumpUtils

- Deleted the `stopShutdownWatchdog` method from SparkSQLEngine to simplify the shutdown process.
- Removed the test for `installShutdownHook` in ThreadDumpUtils, streamlining the test suite by eliminating outdated tests.

Author: wangzhigang1999

externals/kyuubi-spark-sql-engine/src/main/scala/org/apache/kyuubi/engine/spark/SparkSQLEngine.scala

@@ -289,21 +289,6 @@ case class SparkSQLEngine(spark: SparkSession) extends Serverable("SparkSQLEngin
     debug(s"Shutdown Watchdog thread started: ${watchdogThread.getName}")
   }
 
-  /**
-   * Stops the shutdown watchdog thread during normal shutdown process.
-   */
-  private def stopShutdownWatchdog(): Unit = {
-    Option(watchdogThreadRef.getAndSet(null)) match {
-      case Some(thread) if thread.isAlive =>
-        debug("Stopping Shutdown Watchdog")
-        thread.interrupt()
-        info("Shutdown Watchdog stopped - normal shutdown in progress")
-
-      case _ =>
-        debug("Shutdown Watchdog was not running")
-    }
-  }
-
 }
 
 object SparkSQLEngine extends Logging {

kyuubi-common/src/test/scala/org/apache/kyuubi/util/ThreadDumpUtilsSuite.scala

@@ -52,16 +52,6 @@ class ThreadDumpUtilsSuite extends KyuubiFunSuite {
     assert(output.contains("Thread Dump End"))
   }
 
-  test("installShutdownHook should not throw") {
-    try {
-      ThreadDumpUtils.installShutdownHook()
-      assert(true)
-    } catch {
-      case e: Throwable =>
-        fail(s"installShutdownHook should not throw, but got: $e")
-    }
-  }
-
   test("dumpToString should respect showDaemonThreads config") {
     val config = ThreadDumpUtils.DumpConfig(showDaemonThreads = false)
     val dump = ThreadDumpUtils.dumpToString(config)