Inline explicit_operator macro

Now that we require C++11, no need to macro.
diff --git a/src/hb-open-type.hh b/src/hb-open-type.hh
index 453b4fb..c4b54c7 100644
--- a/src/hb-open-type.hh
+++ b/src/hb-open-type.hh
@@ -529,7 +529,7 @@
   unsigned int get_size () const
   { return len.static_size + len * Type::static_size; }
 
-  explicit_operator bool () const { return len; }
+  explicit operator bool () const { return len; }
 
   hb_array_t<      Type> as_array ()       { return hb_array (arrayZ, len); }
   hb_array_t<const Type> as_array () const { return hb_array (arrayZ, len); }