Remove dummy structure declarations for old buggy compilers.

While woolly mammoths still roamed the Earth and before Atlantis
sunk into the ocean, there were C compilers that could not handle
forward structure references, e.g. "struct name;". zlib dutifully
provided a work-around for such compilers. That work-around is no
longer needed, and, per the recommendation of a security audit of
the zlib code by Trail of Bits and TrustInSoft, in support of the
Mozilla Foundation, should be removed since what a compiler will
do with this is technically undefined. From the report: "there is
no telling what interactions the bug could have in the future with
link-time optimizations and type-based alias analyses, both
features that are present (but not default) in clang."

zlib.h

@@ -1759,11 +1759,6 @@ ZEXTERN int ZEXPORT gzgetc_ OF((gzFile file));  /* backward compatibility */
 
 #endif /* !Z_SOLO */
 
-/* hack for buggy compilers */
-#if !defined(ZUTIL_H) && !defined(NO_DUMMY_DECL)
-    struct internal_state {int dummy;};
-#endif
-
 /* undocumented functions */
 ZEXTERN const char   * ZEXPORT zError           OF((int));
 ZEXTERN int            ZEXPORT inflateSyncPoint OF((z_streamp));