Revert "Ubuntu Raspberry Pi: Stop unattended upgrades"

"It turns out that stopping the service is only effective done shortly
after reboot (< 1 minute?). After that a kernel upgrade might be in
progress and the service stops only after the kernel upgrade is
complete. After the upgrade finishes, installation can continue.
However, stopping the unattended kernel upgrade can render the media
unbootable (because /etc/kernetl/postinit.d/zz-deccompress-kernel might
not have run)."

I guess we'll go back to telling people to wait (a possibly long time)
for unattended-upgrades to finish.

This reverts commit d9f50ffa89.

Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #225

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS for Raspberry Pi.rst

@@ -233,12 +233,14 @@ Step 2: Setup ZFS
 
 #. Install ZFS::
 
-     systemctl stop unattended-upgrades
-
      apt update
 
      apt install pv zfs-initramfs
 
+   **Note:** Since this is the first boot, you may get ``Waiting for cache
+   lock`` because ``unattended-upgrades`` is running in the background.
+   Wait for it to finish.
+
 #. Create the root pool:
 
    Choose one of the following options: