Move Context from GSN to utils directory (#2453)

Co-authored-by: Hadrien Croubois <hadrien@openzeppelin.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/token/ERC20/ERC20.sol

@@ -2,7 +2,7 @@
 
 pragma solidity >=0.6.0 <0.8.0;
 
-import "../../GSN/Context.sol";
+import "../../utils/Context.sol";
 import "./IERC20.sol";
 import "../../math/SafeMath.sol";