build: document and cleanup Qt hacks

Summary:

build: convert "echo" usage into a patch in qt package

build: pass XCODE_VERSION through to qt macOS cross compile conf

This should mostly be a no-op, however it would seem to make more sense
that we pass through the XCODE_VERSION we now have in depends, rather
than leaving the version set to 4.3.

build: remove plugin_no_soname from mac qt qmake.conf

plugin_no_soname was removed from Qt some time ago, see upstream commit
1d034244c261520d5e739534dc264c2500e02b5f. It was replaced with
plugin_with_soname, however that is currently only used (as of 5.15.x)
in the Android Clang mkspec.

build: don't copy Info.plist.* into mkspec for macOS qt build

We generate our own Info.plist as part of make deploy, and as far as I
can tell, it doesn't seem to have an effect wether these are present
during qt's build.

I also can't find a single mention of the .app plist in the qt code,
whereas there are multiple instances of .lib.

build: document preprocessing steps in qt package

build: remove global_init_link_order from mac qt qmake.conf

This has been around since the original import of Qt
(38be0d13830efd2d98281c645c3a60afe05ffece), however there
are now only two instatnces of it left in the qt codebase,
and from what I can gather, it's unused.

This is a backport of core#19867

Test Plan: gitian builds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13651