Remove redundant require statements (#1409)

* Remove redundant require statements

Now that SafeMath uses require, the require statements are redundant. They were also previously inconsistent because they were only included in some functions, but not others

* Update ERC20.sol

contracts/token/ERC20/ERC20.sol

@@ -93,8 +93,6 @@ contract ERC20 is IERC20 {
     public
     returns (bool)
   {
-    require(value <= _allowed[from][msg.sender]);
-
     _allowed[from][msg.sender] = _allowed[from][msg.sender].sub(value);
     _transfer(from, to, value);
     return true;
@@ -155,7 +153,6 @@ contract ERC20 is IERC20 {
   * @param value The amount to be transferred.
   */
   function _transfer(address from, address to, uint256 value) internal {
-    require(value <= _balances[from]);
     require(to != address(0));
 
     _balances[from] = _balances[from].sub(value);
@@ -172,6 +169,7 @@ contract ERC20 is IERC20 {
    */
   function _mint(address account, uint256 value) internal {
     require(account != address(0));
+
     _totalSupply = _totalSupply.add(value);
     _balances[account] = _balances[account].add(value);
     emit Transfer(address(0), account, value);
@@ -185,7 +183,6 @@ contract ERC20 is IERC20 {
    */
   function _burn(address account, uint256 value) internal {
     require(account != address(0));
-    require(value <= _balances[account]);
 
     _totalSupply = _totalSupply.sub(value);
     _balances[account] = _balances[account].sub(value);