[SPARK-15386][CORE] Master doesn't compile against Java 1.7 / Process.isAlive

## What changes were proposed in this pull request?

Remove call to Process.isAlive -- Java 8 only. Introduced in https://github.com/apache/spark/pull/13042 / SPARK-15263

## How was this patch tested?

Jenkins tests

Author: Sean Owen <sowen@cloudera.com>

Closes #13174 from srowen/SPARK-15386.