Revert "[iter] Make operator bool explicit"

This reverts commit 66920a6b.

Some of our bots (Oracle Studio and Apple gcc 4.2) do not allow
explicit except for constructors.

https://github.com/harfbuzz/harfbuzz/issues/1127

src/hb-iter-private.hh

@@ -72,7 +72,7 @@ struct Iter<T *>
     array (array_), length (length_) {}
 
   /* Emptiness. */
-  explicit inline operator bool (void) const { return bool (length); }
+  inline operator bool (void) const { return bool (length); }
 
   /* Current item. */
   inline T &operator * (void)