[clangd] Remove LSP command-based #include insertion.

Summary:
clangd will populate #include insertions as addtionalEdits in completion items.

The code completion tests in ClangdServerTest will be added back in D46497.

Reviewers: ilya-biryukov, sammccall

Reviewed By: ilya-biryukov

Subscribers: klimek, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46676

llvm-svn: 332362