Remove possibility to change compile database path at runtime

Summary:
This patch removes the possibility to change the compilation database
path at runtime using the didChangeConfiguration request.  Instead, it
is suggested to use the setting on the initialize request, and clangd
whenever the user wants to use a different build configuration.

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

Differential Revision: https://reviews.llvm.org/D53220

llvm-svn: 344614