Remove libz-host module

We're always using our copy of libz, not the platform's copy anymore. So
always refer to it as libz. The host shared library will still be
libz-host.(so|dylib) because of unique_host_soname.

Test: m host
Change-Id: I4b0fb2b7f1e94834153b83f7a62b8a3e93c8d391