Remove redundant setters and require statements

contracts/staking/contracts/src/interfaces/IStakingEvents.sol

@@ -134,10 +134,4 @@ interface IStakingEvents {
         bytes32 poolId,
         address makerAddress
     );
-
-    /// @dev Emitted by MixinStakingPoolRewardVault when the vault's address is changed.
-    /// @param rewardVaultAddress Address of new reward vault.
-    event StakingPoolRewardVaultChanged(
-        address rewardVaultAddress
-    );
 }

contracts/staking/contracts/src/stake/MixinZrxVault.sol

@@ -27,15 +27,6 @@ import "../immutable/MixinStorage.sol";
 contract MixinZrxVault is
     MixinStorage
 {
-    /// @dev Set the Zrx Vault.
-    /// @param zrxVaultAddress Address of the Zrx Vault.
-    function setZrxVault(address zrxVaultAddress)
-        external
-        onlyOwner
-    {
-        zrxVault = IZrxVault(zrxVaultAddress);
-    }
-
     /// @dev Deposits Zrx Tokens from the `owner` into the vault.
     /// @param owner of Zrx Tokens
     /// @param amount of tokens to deposit.
@@ -43,10 +34,6 @@ contract MixinZrxVault is
         internal
     {
         IZrxVault _zrxVault = zrxVault;
-        require(
-            address(_zrxVault) != address(0),
-            "INVALID_ZRX_VAULT"
-        );
         _zrxVault.depositFrom(owner, amount);
     }
 
@@ -57,10 +44,6 @@ contract MixinZrxVault is
         internal
     {
         IZrxVault _zrxVault = zrxVault;
-        require(
-            address(_zrxVault) != address(0),
-            "INVALID_ZRX_VAULT"
-        );
         _zrxVault.withdrawFrom(owner, amount);
     }
 
@@ -72,10 +55,6 @@ contract MixinZrxVault is
         returns (uint256)
     {
         IZrxVault _zrxVault = zrxVault;
-        require(
-            address(_zrxVault) != address(0),
-            "INVALID_ZRX_VAULT"
-        );
         return _zrxVault.balanceOf(owner);
     }
 }

contracts/staking/contracts/src/staking_pools/MixinStakingPoolRewardVault.sol

@@ -67,16 +67,6 @@ contract MixinStakingPoolRewardVault is
         _;
     }
 
-    /// @dev Sets the address of the reward vault.
-    /// This can only be called by the owner of this contract.
-    function setStakingPoolRewardVault(address payable rewardVaultAddress)
-        external
-        onlyOwner
-    {
-        rewardVault = IStakingPoolRewardVault(rewardVaultAddress);
-        emit StakingPoolRewardVaultChanged(rewardVaultAddress);
-    }
-
     /// @dev Decreases the operator share for the given pool (i.e. increases pool rewards for members).
     /// Note that this is only callable by the pool operator, and will revert if the new operator
     /// share value is greater than the old value.

contracts/staking/contracts/src/sys/MixinParams.sol

@@ -19,6 +19,7 @@
 pragma solidity ^0.5.9;
 
 import "@0x/contracts-utils/contracts/src/LibRichErrors.sol";
+import "@0x/contracts-asset-proxy/contracts/src/interfaces/IAssetProxy.sol";
 import "../immutable/MixinStorage.sol";
 import "../interfaces/IStakingEvents.sol";
 import "../interfaces/IEthVault.sol";