[SPARK-13711][CORE] Don't call SparkUncaughtExceptionHandler in AppClient as it's in driver

## What changes were proposed in this pull request?

AppClient runs in the driver side. It should not call `Utils.tryOrExit` as it will send exception to SparkUncaughtExceptionHandler and call `System.exit`. This PR just removed `Utils.tryOrExit`.

## How was this patch tested?

manual tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11566 from zsxwing/SPARK-13711.

core/src/main/scala/org/apache/spark/deploy/client/AppClient.scala

@@ -125,16 +125,14 @@ private[spark] class AppClient(
       registerMasterFutures.set(tryRegisterAllMasters())
       registrationRetryTimer.set(registrationRetryThread.schedule(new Runnable {
         override def run(): Unit = {
-          Utils.tryOrExit {
-            if (registered.get) {
-              registerMasterFutures.get.foreach(_.cancel(true))
-              registerMasterThreadPool.shutdownNow()
-            } else if (nthRetry >= REGISTRATION_RETRIES) {
-              markDead("All masters are unresponsive! Giving up.")
-            } else {
-              registerMasterFutures.get.foreach(_.cancel(true))
-              registerWithMaster(nthRetry + 1)
-            }
+          if (registered.get) {
+            registerMasterFutures.get.foreach(_.cancel(true))
+            registerMasterThreadPool.shutdownNow()
+          } else if (nthRetry >= REGISTRATION_RETRIES) {
+            markDead("All masters are unresponsive! Giving up.")
+          } else {
+            registerMasterFutures.get.foreach(_.cancel(true))
+            registerWithMaster(nthRetry + 1)
           }
         }
       }, REGISTRATION_TIMEOUT_SECONDS, TimeUnit.SECONDS))