Fixes #3886 by letting qt do the downscaling from the original 256x256 bitmap.
Before/after on Ubuntu and Windows:

Looks good to me.
However, if you do it this way the icon will get encoded into the resource file twice resulting in wasted space.
Please make this change in the source code (https://github.com/bitcoin/bitcoin/blob/master/src/qt/bitcoingui.cpp#L482).
You can then remove the toolbar_*.png from the qrc and from the git repo as it is no longer used.
If deleting from git, they need to be removed from the Makefiles as well. Nuke the entries from src/Makefile.qt.include, that should fix 'make dist', which is causing the build error above.
Yep, I think they should be removed from doc/assets-attribution.md also
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Merged via 8bfdc9a