[SPARK-12667] Remove block manager's internal "external block store" API

This pull request removes the external block store API. This is rarely used, and the file system interface is actually a better, more standard way to interact with external storage systems.

There are some other things to remove also, as pointed out by JoshRosen. We will do those as follow-up pull requests.

Author: Reynold Xin <rxin@databricks.com>

Closes #10752 from rxin/remove-offheap.