fixup! json-writer: print level key in SARIF output

Warning is the default level so does not make any sense to do the
corresponding check.

Author: lzaoral

src/json-writer.cc

@@ -220,8 +220,6 @@ static void sarifEncodeLevel(PTree *result, const std::string &event) {
 
     if (boost::starts_with(event, "error"))
         level = "error";
-    else if (boost::starts_with(event, "warning"))
-        level = "warning";
     else if (boost::starts_with(event, "note"))
         level = "note";