[Frontend] Remove ShowIncludesPretendHeader

It hasn't been written to since https://reviews.llvm.org/D46652, so it
was always empty. I don't have enough context on that diff to know if
the removal of the write to ShowIncludesPretendHeader in that diff was
intentional, but no one's complained about it for five years, so I
assume we're okay to just get rid of it entirely.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D153176