Simplify native's ActiveIn behaviour

[AnnaShaleva]

Native deploy and update code becomes more complicated after #3444. A lot of problems causes nullable native's ActiveIn:

neo-go/pkg/core/interop/context.go

Lines 175 to 177 in 2d4993a

	// ActiveIn returns the hardfork native contract is active starting from or nil in case
	// it's always active.
	ActiveIn() *config.Hardfork

If we use config.HFDefault instead of null ActiveIn and make ActiveIn to return config.Hardfork instead of *config.Hardfork, it will simplify things a lot and allow to unify native deploy/update code.