use mid-grey padlock icon, don't change icon color on appearance change (dark/light mode)

deltachat-ios/Assets.xcassets/ic_lock.imageset/Contents.json

@@ -2,50 +2,17 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "ic_lock_dark_18dp-2.png",
+      "filename" : "ic_lock_lightergrey_1x.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
-      "filename" : "ic_lock_white_18dp-2.png",
-      "appearances" : [
-        {
-          "appearance" : "luminosity",
-          "value" : "dark"
-        }
-      ],
-      "scale" : "1x"
-    },
-    {
-      "idiom" : "universal",
-      "filename" : "ic_lock_dark_18dp.png",
-      "scale" : "2x"
-    },
-    {
-      "idiom" : "universal",
-      "filename" : "ic_lock_white_18dp.png",
-      "appearances" : [
-        {
-          "appearance" : "luminosity",
-          "value" : "dark"
-        }
-      ],
+      "filename" : "ic_lock_lightergrey_2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "ic_lock_dark_18dp-1.png",
-      "scale" : "3x"
-    },
-    {
-      "idiom" : "universal",
-      "filename" : "ic_lock_white_18dp-1.png",
-      "appearances" : [
-        {
-          "appearance" : "luminosity",
-          "value" : "dark"
-        }
-      ],
+      "filename" : "ic_lock_lighergrey_3x.png",
       "scale" : "3x"
     }
   ],

deltachat-ios/Controller/ChatViewController.swift

@@ -847,15 +847,6 @@ extension ChatViewController: MessagesDataSource {
         text.append(imageString)
     }
 
-    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
-        super.traitCollectionDidChange(previousTraitCollection)
-        if #available(iOS 13.0, *) {
-            if self.traitCollection.hasDifferentColorAppearance(comparedTo: previousTraitCollection) {
-                refreshMessages()
-            }
-        }
-    }
-
     func updateMessage(_ messageId: Int) {
         if let index = messageList.firstIndex(where: { $0.id == messageId }) {
             dc_markseen_msgs(mailboxPointer, UnsafePointer([UInt32(messageId)]), 1)