Remove unused declaration

$ corelist version

Data for 2014-04-20
version was first released with perl v5.9.0
remove_tree was only available as of File::Path-2.07
Text::CSV_XS-1.07 is out (and 1.08 is getting close)
Guard Makefile.PL against standalone run with unmet dependencies

When running Makefile.PL manually, not from a cpan client, the required
dependencies are not checked and "weird" errors will show.