Fix/solc 0.7.4 warnings #2391 (#2396)

* Fix unnamed return variable warning

This commit fixes warnings thrown by the solc 0.7.4 compiler:
"Warning: Unnamed return variable can remain unassigned. Add an explicit
return with value to all non-reverting code paths or name the variable."

* Fix function state mutability warning

This commit fixes warnings thrown by the solc 0.7.4 compiler:
"Warning: Function state mutability can be restricted to pure"

* Fix shadows an existing declaration warning

This commit fixes warnings thrown by the solc 0.7.4 compiler:
"Warning: This declaration shadows an existing declaration."

1. Arguments by default are not underscored.
2. If the name isn't available due to shadowing, use prefix underscore.
3. If prefix underscore isn't available due to shadowing, use suffix underscore.

contracts/mocks/GSNRecipientMock.sol

@@ -13,7 +13,7 @@ contract GSNRecipientMock is ContextMock, GSNRecipient {
 
     function acceptRelayedCall(address, address, bytes calldata, uint256, uint256, uint256, uint256, bytes calldata, uint256)
         external
-        view
+        pure
         override
         returns (uint256, bytes memory)
     {