Revert r366458, r366467 and r366468

r366458 is causing test failures. r366467 and r366468 had to be reverted as
they were casuing conflict while reverting r366458.

r366468 [clangd] Remove dead code from BackgroundIndex
r366467 [clangd] BackgroundIndex stores shards to the closest project
r366458 [clangd] Refactor background-index shard loading

llvm-svn: 366551