[clangd] Delete default arguments while moving functions out-of-line

Summary:
Only function declarations should have the default arguments.

This patch makes sure we don't propogate those arguments to out-of-line
definitions.

Fixes https://github.com/clangd/clangd/issues/221

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71187