let containerd_default_runtime be undefined by default

[rptaylor]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
See #9025
Which issue(s) this PR fixes:
Fixes #9025
Special notes for your reviewer:
The default filter is used so the only effect of this change will be to allow users to configure the variable in their inventory better. This is a better design pattern which is already used by other variables such as containerd_snapshotter.
Does this PR introduce a user-facing change?:
No

[k8s-ci-robot]

Hi @rptaylor. Thanks for your PR.

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[cristicalin]

Thanks @rptaylor !

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cristicalin, rptaylor

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[rptaylor]

Thanks! Looks like the CI test is failing for reasons related to the cloud resources ? @cristicalin

[oomichi]

/lgtm