Building on answers by @kyrsquir and @BoBKelso and saving a few reboots.
Reboot into Recovery Mode
Mount of the Data partition using Disk Utility
Open Terminal
Use pkgutil to figure what was installed where on the now mounted Data partition, e.g. pkgutil --files com.apple.pkg.RosettaUpdateAuto --volume /Volumes/Data
Remove Rosetta files from mounted Data partition
Reboot
Verify in Terminal, e.g. whoami on x64 arch should say:
% arch -x86_64 whoami
arch: posix_spawnp: whoami: Bad CPU type in executable
hruan
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