Remove MAP_VARIABLE from hppa bits/mman.h

As suggested in
<https://sourceware.org/pipermail/libc-alpha/2023-February/145890.html>,
remove the MAP_VARIABLE define from the hppa bits/mman.h, for
consistency with Linux 6.2 which removed the define there.

Tested with build-many-glibcs.py for hppa-linux-gnu.