Remove support for dynamic loaders dropped by Dynaloader

[jkahrman]

Remove support for GNU DLD 81a3320

DynaLoader dropped support for GNU DLD in 2013 in Perl 5.19:

"The last release [of GNU DLD] was in 1996, and it has been superseded by dlopen."

https://github.com/Perl/perl5/commit/1e5d7f54016618316299b1aacea865185fab5dd7

Remove NeXT support 5f9bfa5

DynaLoader dropped support for NeXT in 2014 in Perl 5.21

https://github.com/Perl/perl5/commit/f05550c064c2736017a5c65739d9eee325eed149

[jkahrman]

I found a couple more adjacent NeXT references. I've now searched for 'next' not just 'dlsrc'

[jkahrman]

Removed stray semicolon

[Leont]

@bingos can this be merged?