Remove libz.dylib from 'Link Binary with Libraries' build phase

This removes an XCode build warning when building ZipArchive as a
Target Dependency of another project.

According to XCode's build warning, projects that build a static library should not link
directly against dynamic libraries.  Instead, their host projects should link against
their dynamic libraries.  In this case, projects that build ZipArchive as a dependency
should link against libz.dylib in their own 'Link Binary with Libraries' build phase.