[clangd] Remove vscode plugin: now https://github.com/clangd/vscode-clangd
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.
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D78598