Revert "[core] Do not clear custom layers upon style switch"

This reverts commit 8ec875b87c.

src/mbgl/style/style.cpp

@@ -70,9 +70,7 @@ Style::Style(FileSource& fileSource_, float pixelRatio)
 
 void Style::setJSON(const std::string& json, const std::string&) {
     sources.clear();
-    layers.erase(std::remove_if(layers.begin(), layers.end(), [](const auto& layer) {
-            return !layer->template is<CustomLayer>();
-        }), layers.end());
+    layers.clear();
     classes.clear();
 
     StyleParser parser;

test/api/custom_layer.cpp

@@ -90,8 +90,4 @@ TEST(CustomLayer, Basic) {
         }, new TestLayer());
 
     test::checkImage("test/fixtures/custom_layer/basic", test::render(map));
-
-    // Custom layer should be preserved when switching styles
-    map.setStyleJSON(util::read_file("test/fixtures/api/empty.json"), "");
-    test::checkImage("test/fixtures/custom_layer/basic", test::render(map));
 }