do_exec needs no compatibility version in mathoms.c as it's not in the API.

do_exec needs no compatibility version in mathoms.c as it's not in the API.

9555a685dbd794b0 replaced it with a macro and added the full-name version in
mathoms.c to retain compatibility with any program whose source code uses
the full name. However, as do_exec was never in the API, no program would be
using it. (It's also unconditionally explicitly not exported on various
platforms including Win32. Google Codesearch and grep.cpan.me find no users
of it outside the core.)