gui: Remove QT_VERSION fallbacks for Qt < 5

There were surprisingly many `#ifdef` fallbacks for Qt 4.

Remiving them simplifies maintenance, as well as adding new GUI
functionality.

src/qt/peertablemodel.cpp

@@ -64,9 +64,7 @@ public:
 
             interfaces::Node::NodesStats nodes_stats;
             node.getNodesStats(nodes_stats);
-#if QT_VERSION >= 0x040700
             cachedNodeStats.reserve(nodes_stats.size());
-#endif
             for (auto& node_stats : nodes_stats)
             {
                 CNodeCombinedStats stats;