[clangd] Remove codeComplete that returns std::future<>

Summary:
It was deprecated and callback version and is used everywhere.
Only changes to the testing code were needed.

Reviewers: hokein, ioeric, sammccall

Reviewed By: sammccall

Subscribers: mgorny, klimek, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43068

llvm-svn: 324883