Deleted unnecessary import. (#1437)

* Deleted unnecessary import.

* Added back SafeMath.

* Removed another unnecessary import.

contracts/crowdsale/distribution/PostDeliveryCrowdsale.sol

@@ -1,7 +1,6 @@
 pragma solidity ^0.4.24;
 
 import "../validation/TimedCrowdsale.sol";
-import "../../token/ERC20/IERC20.sol";
 import "../../math/SafeMath.sol";
 
 /**

contracts/token/ERC20/SafeERC20.sol

@@ -1,7 +1,7 @@
 pragma solidity ^0.4.24;
 
-import "./ERC20.sol";
 import "./IERC20.sol";
+import "../../math/SafeMath.sol";
 
 /**
  * @title SafeERC20