Removed additionalTag application on unmanaged subnets

[Ankitasw]

What type of PR is this?
/kind bug

What this PR does / why we need it:
This PR removes the adding additional tags from subnets in case of using BYO VPC. This makes the tagging behavior consistent across all the network components(like routetables, natgateways etc.).

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 #3494

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[k8s-ci-robot]

@Ankitasw: This issue is currently awaiting triage.

If CAPA/CAPI contributors determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

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[Ankitasw]

/test pull-cluster-api-provider-aws-e2e

[sedefsavas]

Also, adding a note to external infrastructure doc for this behaviour will be useful. I think there is already a section stating that we add CCM tags for unmanaged resources, but need to clarify additionalTags will not get propagated.

[Ankitasw]

This PR is ready for review.

[Ankitasw]

cc @sedefsavas @richardcase for review.

[Ankitasw]

cc @richardcase @sedefsavas for review

[sedefsavas]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sedefsavas

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