Make Context abstract (#2229)

contracts/GSN/Context.sol

@@ -10,11 +10,7 @@ pragma solidity ^0.6.0;
  *
  * This contract is only required for intermediate, library-like contracts.
  */
-contract Context {
-    // Empty internal constructor, to prevent people from mistakenly deploying
-    // an instance of this contract, which should be used via inheritance.
-    constructor () internal { }
-
+abstract contract Context {
     function _msgSender() internal view virtual returns (address payable) {
         return msg.sender;
     }