[clangd] Don't maintain a list of c-family extensions in vscode extension.

Summary:
There is no need to maintain this list by ourself, vscode already
provides this feature.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63323

llvm-svn: 363372