gitian: remove zlib and libpng deps

Triggered by #3341.

The bitcoin win32 build was relying on an old insecure version of libpng for no good reason. The libqrcode tools make use of libpng but we don't use the tools, just the library.

In turn, this libpng dependency pulled in zlib.

Remove both dependencies both from the documentation and descriptor.

Bump deps version to r10 (for good measure; the used parts of the dependencies don't actually change, so it may not be strictly needed).