Remove testing attribute from Driver.pm

As part of #164, 87752cc and e445acb take out all uses of the `testing`
attribute in Driver.pm. We no longer need it.

lib/Selenium/Remote/Driver.pm

@@ -417,11 +417,6 @@ has 'inner_window_size' => (
 
 );
 
-has 'testing' => (
-    is => 'rw',
-    default => sub { 0 },
-);
-
 sub BUILD {
     my $self = shift;