[Icons] Render lock and dropdown glyphs through the icon engine

Both generators rendered the sentinel SVG to a fixed height x height pixmap
at DPR 1. The pre-migration path rendered larger and let QIcon scale it, so
on a 2x display the games-list lock (13px) and the banner dropdown arrow
(24px) came back crisp; the new path hands the widget a DPR-1 bitmap that
gets upscaled and blurred.

- The games list asks the themed engine for `icons/lock` directly, so the
  item view paints it at the device pixel ratio and the lock also follows
  HighlightedText on a selected row.
- The dropdown goes through `loadThemedIcon(...).pixmap(size, dpr)`, which
  keeps the glyph's aspect ratio and renders at the requested ratio.
- LockPixmapGenerator has no consumer left, so it and its cache are gone.

cockatrice/src/interface/pixel_map_generator.h

@@ -106,19 +106,6 @@ public:
     }
 };
 
-class LockPixmapGenerator
-{
-private:
-    static QMap<int, QPixmap> pmCache;
-
-public:
-    static QPixmap generatePixmap(int height);
-    static void clear()
-    {
-        pmCache.clear();
-    }
-};
-
 class DropdownIconPixmapGenerator
 {
 private: