[clangd] Remove vscode plugin: now https://github.com/clangd/vscode-clangd

Summary:
Moving this out of the monorepo for consistency with other editor plugins.
There's no version lock with clangd itself, and we never ran tests with lit.

The first version from the new repo has been published.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78598