Ubuntu: Remove canmount=noauto on / and /boot

I think (but am not absolutely certain) that things behave as follows:

- The non-zsys initramfs script expects canmount=noauto for the root
  filesystem.  It then mounts the active one (e.g. in the case of
  clones) manually, overriding that.
- zsys manages the canmount attribute.

I am sure that when the system boots with zsys, the initial datasets,
which were created with canmount=noauto, have canmount=on.

Therefore, there seems to be no reason to set canmount=noauto for the
zsys scenario, and I have removed it.  This simplifies the instructions
and may avoid issues like #73.

Signed-off-by: Richard Laager <rlaager@wiktel.com>
Fixes #73

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -594,18 +594,11 @@ Step 3: System Installation
      UUID=$(dd if=/dev/urandom of=/dev/stdout bs=1 count=100 2>/dev/null |
          tr -dc 'a-z0-9' | cut -c-6)
 
-     zfs create -o canmount=noauto -o mountpoint=/ \
+     zfs create -o mountpoint=/ \
          -o com.ubuntu.zsys:bootfs=yes \
          -o com.ubuntu.zsys:last-used=$(date +%s) rpool/ROOT/ubuntu_$UUID
-     zfs mount rpool/ROOT/ubuntu_$UUID
 
-     zfs create -o canmount=noauto -o mountpoint=/boot \
+     zfs create -o mountpoint=/boot bpool/BOOT/ubuntu_$UUID
-         bpool/BOOT/ubuntu_$UUID
-     zfs mount bpool/BOOT/ubuntu_$UUID
-
-   With ZFS, it is not normally necessary to use a mount command (either
-   ``mount`` or ``zfs mount``). This situation is an exception because of
-   ``canmount=noauto``.
 
 #. Create datasets::