win32/libssh2_config.h: Remove hardcoded #define LIBSSH2_HAVE_ZLIB.

Rationale: Everything else in this file states a fact about the win32
platform that is unconditional for that platform.  There is nothing
unconditional about the presence of zlib.  It is neither included with
Windows nor with the platform SDK.  Therefore, this is not an appropriate
place to assert its presence.  Especially as, once asserted, it cannot be
overridden using a compiler flag.

In contrast, if it is omitted, then it can easily be reasserted by adding
a compiler flag defining LIBSSH2_HAVE_ZLIB.

win32/libssh2_config.h

@@ -34,9 +34,6 @@
 #define strcasecmp stricmp
 #endif /* _MSC_VER */
 
-/* Compile in zlib support */
-#define LIBSSH2_HAVE_ZLIB 1
-
 /* Enable newer diffie-hellman-group-exchange-sha1 syntax */
 #define LIBSSH2_DH_GEX_NEW 1