Porting/Maintainers.pl - Remove remaining UPSTREAM => undef entries

There is some agreement that the things left with UPSTREAM => undef (and
the only two other pod/ files that were tracked -- perldtrace and
perlreftut) are not worth tracking in this file anyway, so just remove
them.

I've left s2p and made UPSTREAM => 'blead', as per comments in the first
thread cited below, for now. Maybe it should go too? Other than
regen/warnings.pl there are no other files covered thaty aren't under
cpan/, dist/, ext/ and lib/ ...

http://www.nntp.perl.org/group/perl.perl5.porters/2013/06/msg203768.html
http://www.nntp.perl.org/group/perl.perl5.porters/2013/10/msg208670.html
Porting/Maintainers.pl - Remove unnecessary Maintainers following 0cd7b4f8d7