[clangd] Remove ignored Preamble::CanReuse call from completion

Summary:
Now that the clients who relied on stats for files from preamble are
gone.

Reviewers: ioeric, sammccall

Reviewed By: ioeric

Subscribers: klimek, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47066

llvm-svn: 332976