[clangd] Remove the whitelist std symbols in CanonicalIncludes.

Summary: These symbols have been included via StdSymbolMap.inc.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62527

llvm-svn: 361952