[SPARK-17316][CORE] Fix the 'ask' type parameter in 'removeExecutor'

## What changes were proposed in this pull request?

Fix the 'ask' type parameter in 'removeExecutor' to eliminate a lot of error logs `Cannot cast java.lang.Boolean to scala.runtime.Nothing$`

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #14983 from zsxwing/SPARK-17316-3.