[core] Remove StyleLayer::isBackground()

src/mbgl/map/tile_worker.cpp

@@ -79,7 +79,7 @@ void TileWorker::parseLayer(const StyleLayer& layer, const GeometryTile& geometr
         return;
 
     // Background is a special case.
-    if (layer.isBackground())
+    if (layer.type == StyleLayerType::Background)
         return;
 
     if (!layer.bucket) {

src/mbgl/style/style_layer.cpp

@@ -29,10 +29,6 @@ std::unique_ptr<StyleLayer> StyleLayer::create(StyleLayerType type) {
     }
 }
 
-bool StyleLayer::isBackground() const {
-    return type == StyleLayerType::Background;
-}
-
 bool StyleLayer::hasRenderPass(RenderPass pass) const {
     return bool(passes & pass);
 }

src/mbgl/style/style_layer.hpp

@@ -29,9 +29,6 @@ public:
 
     virtual ~StyleLayer() = default;
 
-    // Determines whether this layer is the background layer.
-    bool isBackground() const;
-
     // Checks whether this layer needs to be rendered in the given render pass.
     bool hasRenderPass(RenderPass) const;