Remove unused prerequisite for IO::String

Also note that IO::String is only necessaryfor Perls before 5.8. Since
Rex requires 5.12, this is unnecessary.

lib/Rex/Commands/Augeas.pm

@@ -51,7 +51,6 @@ use Rex::Commands::Fs;
 use Rex::Commands::File;
 use Rex::Helper::Path;
 use Rex::Helper::Run;
-use IO::String;
 
 my $has_config_augeas = 0;