Remove last lingering unnecessarily exported functions

[abayer]

Changes

fixes #3262

There are a few things left, either by design or uncertainty:

• pkg/apis/pipeline/v1beta1/conversion_error.go's ConvertErrorf - I'm not entirely clear on whether this should be around or not. CannotConvertError is still used in tests in pkg/apis/pipeline/v1alpha1, and the whole thing is just messy enough that I'm not sure what to do.
• pkg/apis/pipeline/v1beta1/version_validation.go's ValidateEnabledAPIFields - it's referenced in docs/developers/README.md and I can't tell if that's something that could be intended to eb used in something depending on Tekton, not just in Tekton itself.
• pkg/pipelinerunmetrics/injection.go and pkg/taskrunmetrics/injection.go's WithInformer - these are both used as function references in their respective .../fake/fake.gos.

/kind cleanup

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[abayer]

/assign @imjasonh @vdemeester

[abayer]

/retest

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[pritidesai]

/lgtm

thanks @abayer

pkg/apis/pipeline/v1beta1/conversion_error.go's ConvertErrorf - I'm not entirely clear on whether this should be around or not. CannotConvertError is still used in tests in pkg/apis/pipeline/v1alpha1, and the whole thing is just messy enough that I'm not sure what to do.

#4281 is removing all the references of the CannotConvertError.

pkg/apis/pipeline/v1beta1/version_validation.go's ValidateEnabledAPIFields - it's referenced in docs/developers/README.md and I can't tell if that's something that could be intended to eb used in something depending on Tekton, not just in Tekton itself.

ValidateEnabledAPIFields is a function which is used internally to check if the enable-api-fields is set to the appropriate version and I think the idea here is to keep this for the pipeline controller and also for the dependencies too.