[clangd] Remove override result handling logic from clangd

Summary:
Since we plan to move handling of override suggestions to Sema with
D52225 this patch just makes sure clangd-side has no logic related to that
anymore and updates tests.

Reviewers: ioeric, ilya-biryukov

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D52226

llvm-svn: 343567