Revert "Introduce Debian Bullseye root on ZFS howto"

This reverts commit b81a471.

We clashed while working on this.  The Bullseye HOWTO in the above
commit was not rebased on the current Buster HOWTO, so a lot of its
delta between Buster and Bullseye is improper.

Signed-off-by: Richard Laager <[email protected]>