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."

deflate.c

@@ -151,10 +151,6 @@ local const config configuration_table[10] = {
 #define EQUAL 0
 /* result of memcmp for equal strings */
 
-#ifndef NO_DUMMY_DECL
-struct static_tree_desc_s {int dummy;}; /* for buggy compilers */
-#endif
-
 /* rank Z_BLOCK between Z_NO_FLUSH and Z_PARTIAL_FLUSH */
 #define RANK(f) (((f) * 2) - ((f) > 4 ? 9 : 0))