Remove prefetch queue since it is legacy

Sources/Networking/ImagePrefetcher.swift

@@ -31,10 +31,6 @@ import AppKit
 import UIKit
 #endif
 
-// The dispatch queue to use for handling resource process, so downloading does not occur on the main thread
-// This prevents stuttering when preloading images in a collection view or table view.
-private let prefetchQueue = DispatchQueue(label: "com.onevcat.Kingfisher.PrefetchQueue")
-
 /// Progress update block of prefetcher. 
 ///
 /// - `skippedResources`: An array of resources that are already cached before the prefetching starting.
@@ -156,45 +152,38 @@ public class ImagePrefetcher {
     /// Starts to download the resources and cache them. This can be useful for background downloading
     /// of assets that are required for later use in an app. This code will not try and update any UI
     /// with the results of the process.
-    public func start()
-    {
-        // Since we want to handle the resources cancellation in the prefetch queue only.
-        prefetchQueue.async {
-            
-            guard !self.stopped else {
-                assertionFailure("You can not restart the same prefetcher. Try to create a new prefetcher.")
-                self.handleComplete()
-                return
-            }
-            
-            guard self.maxConcurrentDownloads > 0 else {
-                assertionFailure("There should be concurrent downloads value should be at least 1.")
-                self.handleComplete()
-                return
-            }
+    public func start() {
+        guard !stopped else {
+            assertionFailure("You can not restart the same prefetcher. Try to create a new prefetcher.")
+            handleComplete()
+            return
+        }
 
-            // Empty case.
-            guard self.prefetchResources.count > 0 else {
-                self.handleComplete()
-                return
-            }
-            
-            let initialConcurrentDownloads = min(self.prefetchResources.count, self.maxConcurrentDownloads)
-            for _ in 0 ..< initialConcurrentDownloads {
-                if let resource = self.pendingResources.popFirst() {
-                    self.startPrefetching(resource)
-                }
+        guard maxConcurrentDownloads > 0 else {
+            assertionFailure("There should be concurrent downloads value should be at least 1.")
+            handleComplete()
+            return
+        }
+
+        // Empty case.
+        guard prefetchResources.count > 0 else {
+            handleComplete()
+            return
+        }
+
+        let initialConcurrentDownloads = min(prefetchResources.count, maxConcurrentDownloads)
+        for _ in 0 ..< initialConcurrentDownloads {
+            if let resource = self.pendingResources.popFirst() {
+                self.startPrefetching(resource)
             }
         }
     }
 
     /// Stops current downloading progress, and cancel any future prefetching activity that might be occuring.
     public func stop() {
-        prefetchQueue.async {
-            if self.finished { return }
-            self.stopped = true
-            self.tasks.values.forEach { $0.cancel() }
-        }
+        if finished { return }
+        stopped = true
+        tasks.values.forEach { $0.cancel() }
     }
     
     func downloadAndCache(_ resource: Resource) {
@@ -270,13 +259,11 @@ public class ImagePrefetcher {
     }
     
     func reportCompletionOrStartNext() {
-        prefetchQueue.async {
-            if let resource = self.pendingResources.popFirst() {
-                self.startPrefetching(resource)
-            } else {
-                guard self.tasks.isEmpty else { return }
-                self.handleComplete()
-            }
+        if let resource = self.pendingResources.popFirst() {
+            startPrefetching(resource)
+        } else {
+            guard tasks.isEmpty else { return }
+            handleComplete()
         }
     }