Governance: Remove legacy CreateGovernance instructions

[SebastianBor]

Summary

Remove legacy instructions to create asset specific governances: CreateProgramGovernance, CreateTokenGovernance and CreateMintGovernance. The instructions are leftovers from the spl-governance inception time when the governance accounts were top level accounts and Realm account didn't exist. The asset specific accounts are no longer required because the generic Governance account can handle all the use cases.

Backward compatibility

The change is backward compatible from the state point of view and all the asset specific Governance accounts created in previous versions are still supported.

Any program or client using the assets specific instructions to create Governance accounts must switch to CreateGovernance instruction and explicitly transfer the relevant authorities to the new Governance.

Further work

There is no concept of governed_account any longer and in a follow up PR the field on Governance account will be renamed to governance_seed to indicate its actual meaning.

[joncinque]

Looks great!