Update the "Mark as" icons. Fixes #3687

Mac/AppAssets.swift

@@ -147,7 +147,7 @@ struct AppAssets {
 	}
 
 	static var markAllAsReadImage: RSImage = {
-		return RSImage(named: "markAllAsRead")!
+		return NSImage(systemSymbolName: "arrow.up.arrow.down.circle", accessibilityDescription: nil)!
 	}()
 
 	static var nextUnreadImage: RSImage = {

Mac/Resources/Assets.xcassets/markAllAsRead.imageset/Contents.json

@@ -1,16 +0,0 @@
-{
-  "images" : [
-    {
-      "filename" : "markAllAsRead.pdf",
-      "idiom" : "universal"
-    }
-  ],
-  "info" : {
-    "author" : "xcode",
-    "version" : 1
-  },
-  "properties" : {
-    "preserves-vector-representation" : true,
-    "template-rendering-intent" : "template"
-  }
-}