🏃 Remove WaitGroup for MachineSet controller deletion requests

[akhil-rane]

What this PR does / why we need it:
This PR removes the WaitGroup for MachineSet controller deletion requests.
It was added around v1alpha1 timeframe because Delete at that time was blocking.
Today, these Delete calls can be done sequentially because they return right
away and don't have to wait for the delete to finish.

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 #2989

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[fabriziopandini]

thanks @akhil-rane , good catch!
/ok-to-test
/approve

[fabriziopandini]

/hold
Oh, I just noticed that #3089 is tackling this problem as part of a broader cleanup 😓
@vincepri how do you want to proceed here?

[akhil-rane]

@fabriziopandini Oops! my bad. I did not notice that @vincepri had already created a PR.

[vincepri]

/retitle 🏃 Remove WaitGroup for MachineSet controller deletion requests

[vincepri]

/hold cancel
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: akhil-rane, fabriziopandini, vincepri

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[vincepri]

@fabriziopandini We can merge this one first, then I'll rebase and we merge the other one in

[vincepri]

/milestone v0.3.7
/lgtm