`@0x/contracts-staking`: Merge `exchange_fees` unit tests into `protocol_fees` unit tests. `@0x/contracts-staking`: Remove `ProtocolFeeActor` and any use of it. `@0x/contracts-staking`: Remove unused constants. `@0x/contracts-staking`: Move WETH assertion constructor into `MixinDeploymentConstants`. `@0x/contracts-staking`: Add more unit tests.