- Remove Akka dependency from core. Note: the streaming-akka project still uses Akka.
- Remove HttpFileServer
- Remove Akka configs from SparkConf and SSLOptions
- Rename `spark.akka.frameSize` to `spark.rpc.message.maxSize`. I think it's still worth to keep this config because using `DirectTaskResult` or `IndirectTaskResult`  depends on it.
- Update comments and docs

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #10854 from zsxwing/remove-akka.