Fix #4731 ("Reset cover to default" does not work properly)

Changelog.yaml

@@ -51,6 +51,9 @@
     - title: "Fix error while sending book with non-ascii character in title/author to device on linux"
       tickets: [4690]
 
+    - title: "Fix reset cover in edit meta information dialog does not actually remove cover"
+      tickets: [4731]
+
   new recipes:
     - title: Kamera Bild
       author: Darko Miletic

src/calibre/gui2/dialogs/metadata_single.py

@@ -653,8 +653,11 @@ class MetadataSingleDialog(ResizableDialog, Ui_MetadataSingleDialog):
             d = d + self.local_timezone_offset
             self.db.set_timestamp(self.id, d)
 
-            if self.cover_changed and self.cover_data is not None:
-                self.db.set_cover(self.id, self.cover_data)
+            if self.cover_changed:
+                if self.cover_data is not None:
+                    self.db.set_cover(self.id, self.cover_data)
+                else:
+                    self.db.remove_cover(self.id)
         except IOError, err:
             if err.errno == 13: # Permission denied
                 fname = err.filename if err.filename else 'file'