Latest commit b03e606 …
They weren't updated in the last 6 years and aren't helpful anymore (e.g. by causing unrelated matches during git grep searches).
Unmodified sources for zlib-1.2.5 have been included with Mono, this is used to support compression and uncompression in older Linux systems that might have shipped older versions of zlib and to reduce dependencies. They were imported in Oct 22 2010.