Remove ppport.h's broken re-implementation of eval_pv().

Remove ppport.h's broken re-implementation of eval_pv().

Recent versions of Devel::PPPort try to redefine eval_pv() to
dodge a bug in pre-5.31 Perl versions.  Unfortunately the redefinition
fails on compilers that don't support statements nested within
expressions.  However, we aren't actually interested in this bug fix,
since we always call eval_pv() with croak_on_error = FALSE.
So, until there's an upstream fix for this breakage, just comment
out the macro to revert to the older behavior.

Per report from Wei Sun, as well as previous buildfarm failure
on pademelon (which I'd unfortunately not looked at carefully
enough to understand the cause).  Back-patch to all supported
versions, since we're using the same ppport.h in all.

Discussion: https://postgr.es/m/tencent_2EFCC8BA0107B6EC0F97179E019A8A43C806@qq.com
Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=pademelon&dt=2022-02-02%2001%3A22%3A58