let Bison generate the header directly

let Bison generate the header directly

Forwarded: not-yet
Last-Update: 2020-08-04

Starting with Bison 3.7, the generated C++ file #include's the header
by default, instead of duplicating it. So we should not delete it.

Remove the code to add #ifdef guards to the header, since Bison adds
them itself since version 2.6.3.

Gbp-Pq: Name bison_3.7.diff