[clangd] Remove didOpen extraFlags extension.

Summary:
This was added in D34947 to support YCM, but YCM actually provides *all* args,
and this was never actually used.
Meanwhile, we grew another extension that allows specifying all args.

I did find one user of this extension: https://github.com/thomasjo/atom-ide-cpp.
I'll reach out, there are multiple good alternatives:
 - compile_commands.txt can serve the same purpose as .clang_complete there
 - we can add an extension to support setting the fallback command

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53641

llvm-svn: 345969