[clang-tidy] remove default header-filter for run-clang-tidy

Summary:
run-clang-tidy.py was enforcing '-header-filter' parameter with an
unfortunate default value when none was given. Thus, leading to
overwritten clang-tidy configuration (e.g. from .clang-tidy).

This change removes the default value for '-header-filter' resulting in
the default behaviour of clang-tidy itself.

Fixes PR#41426

Reviewed By: hintonda

Patch by Torbjörn Klatt!

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D61747

llvm-svn: 361344