Makefile: use 'rm -f' for verify-boilerplate

[invidian]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

So the target is idempotent and it does not print to stderr on CI.

Example: https://prow.k8s.io/view/gs/kubernetes-jenkins/pr-logs/pull/kubernetes-sigs_cluster-api-provider-aws/3344/pull-cluster-api-provider-aws-verify/1518490957455036416#1:build-log.txt%3A16

[k8s-ci-robot]

@invidian: 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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[sedefsavas]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sedefsavas

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• OWNERS [sedefsavas]

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[invidian]

Rebased to fix CI.

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[k8s-ci-robot]

@invidian: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-aws-verify	05ec969	link	true	/test pull-cluster-api-provider-aws-verify

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.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[Ankitasw]

@invidian could you please run make generate again and push the change?

[sedefsavas]

Verify failure is not related to this PR, merging.

[invidian]

@invidian could you please run make generate again and push the change?

I did, but it didn't change anything :(

Verify failure is not related to this PR, merging.

Thanks :)