[MINOR][CORE] Fix a HadoopRDD log message and remove unused imports in rdd files.

## What changes were proposed in this pull request?

This PR fixes the following typos in log message and comments of `HadoopRDD.scala`. Also, this removes unused imports.
```scala
-      logWarning("Caching NewHadoopRDDs as deserialized objects usually leads to undesired" +
+      logWarning("Caching HadoopRDDs as deserialized objects usually leads to undesired" +
...
-      // since its not removed yet
+      // since it's not removed yet
```

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13294 from dongjoon-hyun/minor_rdd_fix_log_message.