[SPARK-3674] [EC2] Clear SPARK_WORKER_INSTANCES when using YARN

cc andrewor14

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6424 from shivaram/spark-worker-instances-yarn-ec2 and squashes the following commits:

db244ae [Shivaram Venkataraman] Make Python Lint happy
0593d1b [Shivaram Venkataraman] Clear SPARK_WORKER_INSTANCES when using YARN

(cherry picked from commit d3e026f8)
Signed-off-by: Andrew Or <andrew@databricks.com>