Revert "Invalid viewkeys just return isvalid=false"

This reverts commit 309ba4a5bf.

src/test/rpc_wallet_tests.cpp

@@ -333,6 +333,8 @@ BOOST_AUTO_TEST_CASE(rpc_wallet_z_validateviewingkey)
     UniValue resultObj = retValue.get_obj();
     bool b = find_value(resultObj, "isvalid").get_bool();
     BOOST_CHECK_EQUAL(b, false);
+    b = find_value(resultObj, "ismine").get_bool();
+    BOOST_CHECK_EQUAL(b, false);
 }
 
 /**