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/bantablemodel.cpp

@@ -52,9 +52,7 @@ public:
         node.getBanned(banMap);
 
         cachedBanlist.clear();
-#if QT_VERSION >= 0x040700
         cachedBanlist.reserve(banMap.size());
-#endif
         for (const auto& entry : banMap)
         {
             CCombinedBan banEntry;