⚠️ Remove clusterctl management groups

[fabriziopandini]

What this PR does / why we need it:
This PR removes the --management-group flag from clusterctl, the corresponding field from the clusterctl library, and a big chunk of the logic related to having many instances of a provider into a management cluster (not supported anymore).

Please note that more cleanup should follow, as described in #4056 (comment), but I'm breaking down the change in smaller PRs in order to make review easier

Which issue(s) this PR fixes:
Rif #4056

[vincepri]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

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[fabriziopandini]

/retest

[k8s-ci-robot]

@fabriziopandini: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-cluster-api-apidiff-main	aba35d7	link	/test pull-cluster-api-apidiff-main

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