[SPARK-14102][CORE] Block `reset` command in SparkShell

## What changes were proposed in this pull request?

Spark Shell provides an easy way to use Spark in Scala environment. This PR adds `reset` command to a blocked list, also cleaned up according to the Scala coding style.
```scala
scala> sc
res0: org.apache.spark.SparkContext = org.apache.spark.SparkContext718fad24
scala> :reset
scala> sc
<console>:11: error: not found: value sc
       sc
       ^
```
If we blocks `reset`, Spark Shell works like the followings.
```scala
scala> :reset
reset: no such command.  Type :help for help.
scala> :re
re is ambiguous: did you mean :replay or :require?
```

## How was this patch tested?

Manual. Run `bin/spark-shell` and type `:reset`.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11920 from dongjoon-hyun/SPARK-14102.

repl/scala-2.11/src/main/scala/org/apache/spark/repl/SparkILoop.scala

@@ -19,12 +19,11 @@ package org.apache.spark.repl
 
 import java.io.BufferedReader
 
-import Predef.{println => _, _}
-import scala.util.Properties.{javaVersion, versionString, javaVmName}
-
-import scala.tools.nsc.interpreter.{JPrintWriter, ILoop}
+import scala.Predef.{println => _, _}
 import scala.tools.nsc.Settings
+import scala.tools.nsc.interpreter.{ILoop, JPrintWriter}
 import scala.tools.nsc.util.stringFromStream
+import scala.util.Properties.{javaVersion, javaVmName, versionString}
 
 /**
  *  A Spark-specific interactive shell.
@@ -75,11 +74,9 @@ class SparkILoop(in0: Option[BufferedReader], out: JPrintWriter)
     echo("Type :help for more information.")
   }
 
-  import LoopCommand.{ cmd, nullary }
-
-  private val blockedCommands = Set("implicits", "javap", "power", "type", "kind")
+  private val blockedCommands = Set("implicits", "javap", "power", "type", "kind", "reset")
 
-  /** Standard commands **/
+  /** Standard commands */
   lazy val sparkStandardCommands: List[SparkILoop.this.LoopCommand] =
     standardCommands.filter(cmd => !blockedCommands(cmd.name))
 
@@ -112,9 +109,9 @@ object SparkILoop {
         val output = new JPrintWriter(new OutputStreamWriter(ostream), true)
         val repl = new SparkILoop(input, output)
 
-        if (sets.classpath.isDefault)
+        if (sets.classpath.isDefault) {
           sets.classpath.value = sys.props("java.class.path")
-
+        }
         repl process sets
       }
     }