Drop pkg/apihelper

[marquiz]

What would you like to be added:

Get rid of the pkg/apihelper package (and the related mocks) and replace it with direct usage of client(set) libraries.

This also means that the nfd-master unit tests need a major overhaul. Which is actually good as in their current form they are almost unmaintainable. E.g. error messages from the mock clients are practically indecipherable. Also, the current pattern of verifying that some specific set of k8s api calls, in particular order, with exact parameters, are made is very impractical if not pointless, and makes refactoring/modifying certain parts of the code very painful (and these are practically covered by the e2e tests).

Why is this needed:

Simplify code. Also uniformity accross the code base: many of the apiserver access (e.g. NFD API, NRT CRs) is already done directly, without resorting to apihelper.