[clangd] Remove caching of compilation database commands.

Summary:
The CDB implementations used in open-source code are fast, and our private
slow CDB will soon do the relevant caching itself.

Simplifying the GlobalCDB layer in clangd is important to get auto-index
implemented at the right layer.

Reviewers: ioeric, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53439

llvm-svn: 345024