install: zfs: fix setting compression to 'off'

install: zfs: fix setting compression to 'off'

The default for the `compression` property in ZFS got changed ~2 years
ago by

    56fa4aa96 ("Default to ON for compression") [0]

Support for setting this option originally got introduced into the
installer in 2016 by

    c7779156 ("refactor disk setup, add advanced ZFS options") [1]

where the default of 'off' was still correct.

As the installer only set the property if it was *not* explicitly set
to 'on', this actually regressed in the meantime.

Thus just remove the conditional all together, as the definedness-check
did not have any impact anyway (since $value gets set to 'on'
regardless) and the latter just causes regressions like this one.

Tested by installing once w/o the patch to confirm the report and once
with the patch applied, checking `zfs get compression` on the freshly
installed system.

[0] https://github.com/openzfs/zfs/commit/56fa4aa96eb3875f254e93eaef646ea20ba187f9
[1] https://git.proxmox.com/?p=pve-installer.git;a=commit;h=c7779156db5c38cf184e143de0cab534bd0a9cb1

Reported-by: Friedrich Weber <f.weber@proxmox.com>
Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>