Make hb-features.h usable standalone

The intended use for hb-features.h is to
be included standalone, so we can't put
the single-include guards in here.

src/hb-features.h.in

@@ -22,15 +22,9 @@
  * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
  */
 
-#if !defined(HB_H_IN) && !defined(HB_NO_SINGLE_HEADER_ERROR)
-#error "Include <hb.h> instead."
-#endif
-
 #ifndef HB_FEATURES_H
 #define HB_FEATURES_H
 
-#include "hb-common.h"
-
 HB_BEGIN_DECLS