Remove the zlib CFLAGS and LDFLAGS settings from the xcode project file.

We're linking against libcompression all the time now, we don't need to
fall back to zlib.  zlib support will still be used when lldb is built on
linux et al systems, so I'm not removing any of the source support, but
when built on darwin with xcode, we'll be using libcompression.

llvm-svn: 349572