[q3map2] do not copypaste zlib, fix 96

unzip.c contains many bits copypasted from an older zlib which mismatches with newer zlib loaded by newer libpng.

That fix just delete this zlib stuff and makes unzip.c relying on zlib the same way lipbng does. Fixes #96 (closed).

You know you need this fix if q3map2 spits out these lines while compiling maps against png textures (invalidating all the related shaders):

WARNING: An error occurred reading PNG image
libpng error: bad parameters to zlib

Thanks to `Ishq who figured out the issue.