common-instancetypes: Remove FG and update docs

[lyarwood]

/cc @jcanocan

What this PR does / why we need it:

The FG controlling the common-instancetypes deployment feature has GA'd with KubeVirt v1.4.0 so remove the reference and update our docs.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

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• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Testing: New code requires new unit tests. New features and bug fixes require at least on e2e test
• Documentation: A user-guide update was considered and is present (link) or not required. You want a user-guide update if it's a user facing feature / API change.
• Community: Announcement to kubevirt-dev was considered

Release note:

NONE

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[jcanocan]

/lgtm

[lyarwood]

/cc @aburdenthehand

Would you mind reviewing this if you have time?

[kubevirt-bot]

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