⚠️ Remove clusterctl --watching-namespace

[fabriziopandini]

What this PR does / why we need it:
This PR remove the --watching-namespace from clusterctl and the corresponding field from the clusterctl library, thus starting to remove the options allowing the users to create a management cluster with multiple instances of the same provider, which is a configuration not supported anymore by the tool.

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

[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	0d4903f	link	/test pull-cluster-api-apidiff-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[fabriziopandini]

/test pull-cluster-api-test-main

[vincepri]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

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