Remove unused class JsonQuery (#3017)

Introduced 3 years ago in c2e2dfb, not used ever since. Safe to remove.

src/util/JsonQuery.cpp

@@ -1,9 +0,0 @@
-#include "JsonQuery.hpp"
-
-namespace chatterino {
-
-JsonQuery::JsonQuery()
-{
-}
-
-}  // namespace chatterino

src/util/JsonQuery.hpp

@@ -1,12 +0,0 @@
-#pragma once
-
-class QJsonObject;
-
-namespace chatterino {
-class JsonQuery
-{
-public:
-    JsonQuery();
-};
-
-}  // namespace chatterino