Remove duplicate gettext_noop() definition

It's defined in gettext.h so no need for us to duplicate the work.
diff --git a/vncviewer/i18n.h b/vncviewer/i18n.h
index bf5a50c..0ce9e1b 100644
--- a/vncviewer/i18n.h
+++ b/vncviewer/i18n.h
@@ -23,7 +23,6 @@
 #include "gettext.h"
 
 #define _(String) gettext (String)
-#define gettext_noop(String) String
 #define N_(String) gettext_noop (String)
 
 #endif /* _I18N_H */