Minor fix

The value "env" is never used in SparkContext.scala.
Add detailed comment for method setDelaySeconds in MetadataCleaner.scala instead of the unsure one.

Author: WangTao <barneystinson@aliyun.com>

Closes #1105 from WangTaoTheTonic/master and squashes the following commits:

688358e [WangTao] Minor fix
rxin's comments.
Merge remote-tracking branch 'upstream/master' into SPARK-2177