Porting/Maintainers.pl - Remove the remaining MAINTAINER fields

Porting/Maintainers.pl - Remove the remaining MAINTAINER fields

This changes all six modules concerned (B::Concise, B::Deparse, VMS::DCLsym,
VMS::Stdio, warnings and Win32CORE) to P5P maintainership, but that is the
case in practice anyway (since they all live in ext/ or lib/, apart from
B::Deparse, which is surely in error in being in dist/ since it has no CPAN
dual-life distribution).

This was discussed on p5p and #onionsketch:
http://www.nntp.perl.org/group/perl.perl5.porters/2013/06/msg203768.html
http://www.nntp.perl.org/group/perl.perl5.porters/2013/10/msg208704.html
http://irclog.perlgeek.de/onionsketch/2013-10-17