Subnet registry storage clean up

[cryptoAtwill]

There are fields in SubnetRegistryStorage related to rewards, two of them are

ipc/contracts/contracts/lib/LibSubnetRegistryStorage.sol

Line 15 in bcc73c9

address SUBNET_ACTOR_REWARD_FACET;

and

ipc/contracts/contracts/lib/LibSubnetRegistryStorage.sol

Line 31 in bcc73c9

bytes4[] subnetActorRewarderSelectors;

these should be removed as it's for collateral withdraw only, part of a legacy code and only for collateral based power mode. It's not really at the facet level. It should not be confused with the validator reward facet from activity. It should be removed in the upcoming contract refactor.