[LegacyPassManager] Delete BasicBlockPass/Manager.

Summary:
Delete the BasicBlockPass and BasicBlockManager, all its dependencies and update documentation.
The BasicBlockManager was improperly tested and found to be potentially broken, and was deprecated as of rL373254.

In light of the switch to the new pass manager coming before the next release, this patch is a first cleanup of the LegacyPassManager.

Reviewers: chandlerc, echristo

Subscribers: mehdi_amini, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69121