[clangd] clangd-indexer: Drop support for MR-via-YAML

Summary:
It's slow, and the open-source reduce implementation doesn't scale properly.
While here, tidy up some dead headers and comments.

Reviewers: kadircet

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D52517

llvm-svn: 343759