fix: remove modification of networkinterface for ipv6

[Skarlso]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Eventually, AWS' ipv6 docs aren't quite sure and eksctl completely removed dabbling with the network interface in launch templates. We are going to follow suit.

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 #2420 (comment)

Special notes for your reviewer:

Checklist:

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

Release note:

Remove setting `Ipv6Prefix: aws.String("auto")` for network interfaces when IPv6 is enabled.

[Skarlso]

/test ?

[k8s-ci-robot]

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/test ?

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[Skarlso]

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

[Ankitasw]

/lgtm

cc @richardcase

[Skarlso]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Skarlso

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