[q3map2] more zlib deduplication

z reported this issue on IRC:

[ 57%] Building C object tools/quake3/CMakeFiles/q3data.dir/common/unzip.c.o
/home/z/xonotic/xonotic/netradiant/tools/quake3/common/unzip.c:2360:8: error: redefinition of ‘struct internal_state’
 struct internal_state {
        ^
In file included from /home/z/xonotic/xonotic/netradiant/tools/quake3/common/unzip.h:22:0,
                 from /home/z/xonotic/xonotic/netradiant/tools/quake3/common/unzip.c:39:
/usr/include/zlib.h:83:8: note: originally defined here
 struct internal_state;
        ^
tools/quake3/CMakeFiles/q3data.dir/build.make:254: recipe for target 'tools/quake3/CMakeFiles/q3data.dir/common/unzip.c.o' failed
make[2]: *** [tools/quake3/CMakeFiles/q3data.dir/common/unzip.c.o] Error 1

It looks like there was more stuff from zlib to delete.

The whole deleted part was introduced by these words:

/* WARNING: this file should *not* be used by applications. It is
   part of the implementation of the compression library and is
   subject to change. Applications should only use zlib.h.
 */

And should never have been there.

z successfully tested this change on his own system, and I also made some maps compilation involving unzip and png to validate this.