[core] Remove internal threading from OnlineFileRequest

There's no need to do the work that OnlineFileRequest does on a separate thread from the DefaultFileSource thread, and having AsyncTasks proxy to other tasks across a thread boundary adds needless complexity.

test/map/map_context.cpp

@@ -6,10 +6,13 @@
 #include <mbgl/platform/default/headless_display.hpp>
 #include <mbgl/storage/online_file_source.hpp>
 #include <mbgl/util/thread.hpp>
+#include <mbgl/util/run_loop.hpp>
 
 using namespace mbgl;
 
 TEST(MapContext, DoubleStyleLoad) {
+    util::RunLoop loop;
+
     std::shared_ptr<HeadlessDisplay> display = std::make_shared<HeadlessDisplay>();
     HeadlessView view(display, 1, 512, 512);
     OnlineFileSource fileSource;