Don't list cperl in default `perlbrew available`

[Grinnz]

There have been continual instances of people getting confused by cperl being listed in perlbrew available and installing it, then complaining because things don't work, without realizing that they are not using a standard Perl. Not least because there have been periods of time (perhaps currently?) where standard Perls are not listed due to bugs in scraping cpan.org. cperl is not Perl and should not be included by default confusing newcomers.

Ref: https://www.reddit.com/r/perl/comments/9qffgl/is_my_perl_install_hosed/, https://github.com/gugod/App-perlbrew/issues/620, https://rt.cpan.org/Ticket/Display.html?id=125553