[C++20] [Modules] Remove unmaintained Header Module

Currently there is a -emit-header-module mode, which can combine several
headers together as a module interface. However, this breaks our
assumption (for standard c++ modules) about module interface. The module
interface should come from a module interface unit. And if it is a
header, it should be a header unit. And currently we have no ideas to
combine several headers together.

So I think this mode is an experimental one and it is not maintained and
it is not used. So it will be better to remove them.

Reviewed By: Bigcheese, dblaikie, bruno

Differential Revision: https://reviews.llvm.org/D137609