build: don't compress macOS DMG

Summary:
Skip compressing the macOS DMG, and drop related build steps and dependencies. Uncompressed the DMG increases from ~16mb to ~30mb, which compared to other software a user may download, (Firefox 125mb, VLC 52mb, Open Office 176mb), is still relatively small.Note that if / when we build with LTO enabled for releases, this size will shrink back down significantly again.

native_libdmg-hfsplus is not maintained, and I doubt the DMG creation feature will ever be fixed. If at some point xorrisofs supports compressing dmgs, we could enable that.

This is a backport of core#24031

This is one step needed to make the MacOS build work with the current guix script.

Test Plan:
gitian-osx

HOSTS="x86_64-apple-darwin" contrib/guix/guix-build
This still fails with an exotic clang related error ld: unrecognised emulation mode: acosx_version_min, but it no longer fails with a cannot find lz error.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13950