refactor: Remove usage of non-meta handlers

[dkoshkin]

Remove code for mutation handlers that was used to support individual handlers. As we are only registering the meta handlers, this makes it simpler to add new handlers and only a single place to add variable unit tests.

I also removed this assertion since the meta registration code will already fail if Mutate is not implemented and mutation.GeneratePatches and commonhandlers.Named interfaces are no needed if registering in the meta handler.

	_ commonhandlers.Named     = &...PatchHandler{}
	_ mutation.GeneratePatches = &...PatchHandler{}
	_ mutation.MetaMutator     = &...PatchHandler{}

[supershal]

Thank you for the clean up