[SPARK-6846] [WEBUI] [HOTFIX] return to GET for kill link in UI since YARN AM won't proxy POST

Partial undoing of SPARK-6846; YARN AM proxy won't forward POSTs, so go back to GET for kill links in Spark UI. Standalone UIs are not affected.

Author: Sean Owen <sowen@cloudera.com>

Closes #5837 from srowen/SPARK-6846.2 and squashes the following commits:

c17c386 [Sean Owen] Partial undoing of SPARK-6846; YARN AM proxy won't forward POSTs, so go back to GET for kill links in Spark UI. Standalone UIs are not affected.