Cleanup the api packages to remove unused duck templates

[slinkydeveloper]

Describe the bug
In a previous PR #4652 I've found out there are some types we don't need anymore between the templates, some of them probably doesn't even need to be versioned following the k8s convention.

Expected behavior
Cleanup that code

/assign

[lberk]

@slinkydeveloper just looking at the age of the issue, is this something you're actively working on? (trying to determine the best milestone to put this in)

[lberk]

also, just a quick word of caution (via Scott), lets be careful with what we remove considering the number of places eventing is vendor'd (both what we know, knative-sandbox) as well as others we don't know about.

[slinkydeveloper]

I already fixed that, but i forgot to link the PR: #4749

/close

[knative-prow-robot]

@slinkydeveloper: Closing this issue.

In response to this:

I already fixed that, but i forgot to link the PR: #4749

/close

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