## 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.