Drop unused zlib header.

--- apngdis-2.9-src/apngdis.cpp
+++ apngdis-2.9-src/apngdis.cpp
@@ -32,7 +32,6 @@
 #include <string.h>
 #include <vector>
 #include "png.h"     /* original (unpatched) libpng is ok */
-#include "zlib.h"
 
 #define notabc(c) ((c) < 65 || (c) > 122 || ((c) > 90 && (c) < 97))