[Bug preprocessor/45599] Remove all code applying to obsolete "#pragma once"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45599

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'd just go for RESOLVED INVALID. We don't want to remove the code that
implements a supported feature.