Excise zlib from deps/Makefile

We don't actually build zlib on any platforms anymore, right?  I assume that's what [these](https://github.com/JuliaLang/julia/pull/8186/files#diff-3ba529ae517f6b57803af0502f52a40bL89) [lines](https://github.com/JuliaLang/julia/pull/8186/files#diff-b67911656ef5d18c4ae36cb6741b7965L182) being commented out means anyway, unless someone's manually calling `make -C deps install-zlib` which.... probably shouldn't be happening. :P