Remove s2p from Porting/Maintainers.pl

As per RJBS's instructions in the onionsketch:
http://irclog.perlgeek.de/onionsketch/2013-10-17
Porting/Maintainers.pl - Remove most UPSTREAM flags and generate from FILES

With only one exception (version, and only until an outstanding rt.cpan.org
ticket is resolved), UPSTREAM is now 'cpan' for all FILES in cpan/, and
'blead' for everything else, so there is no point in explicitly stating
this: remove them all and auto-generate the same information instead.

The was discussed on p5p in the following thread:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2013-10/msg00321.html