Clean up the [un]structured conversions a bit

[jcrossley3]

I think this is "The Right Way" to do it, but I could be wrong. Makes
the code a bit smaller anyway.

Fixes #20 (comment)

[knative-prow-robot]

@jcrossley3: 0 warnings.

Details

In response to this:

I think this is "The Right Way" to do it, but I could be wrong. Makes
the code a bit smaller anyway.

Fixes #20 (comment)

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[trshafer]

Thanks for doing this!

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jcrossley3, trshafer

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-serving-operator-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/knativeserving/common/images.go	84.1%	86.2%	2.1

[trshafer]

/lgtm