Do not warn about enabled ABIEncoderV2 anymore.

test/cmdlineTests/abiencoderv2_no_warning/input.sol

@@ -0,0 +1,7 @@
+pragma solidity >=0.0;
+pragma experimental ABIEncoderV2;
+contract C {
+    struct S { uint x; }
+    function f(S memory) public pure {
+    }
+}