Remove nullable hack when API version upgrades

[mateusoliveira43]

As described in #1261 (comment) OADP should not add nullable keys to CRDs' YAML files

Solution would be

• remove hack from Makefile (seems the right one, as motivation was another tool that uses OADP needing that), as tried in feat: Nullable logic in code #1261
• transform necessary fields in pointers, so they can be nil in Go (and null in YAML) (also removing hack)