Suppress the '- default -' selection of Print/Filament/Printer settings

if there is any valid option available. This feature has been asked for
by the Prusa3D customers as the '- default -' option has nothing to do
with the actual printer and when selected accidentally, it could even
lead to a printer damage.

lib/Slic3r/Config.pm

@@ -36,6 +36,7 @@ sub new_from_defaults {
     my (@opt_keys) = @_;
     
     my $self = $class->new;
+    # Instantiating the C++ class Slic3r::FullPrintConfig.
     my $defaults = Slic3r::Config::Full->new;
     if (@opt_keys) {
         $self->set($_, $defaults->get($_))