[clangd] Disable ExtractVariable for C

Summary:
Currently extract variable doesn't spell the type explicitly and just
uses an `auto` instead, which is not available in C.

Reviewers: usaxena95

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75053