remove unused dc_empty_server() and related code

'Delete emails from server' was an experimental ad-hoc function
that was added before 'Automatically delete old messages' was introduced
to allow at least some way to cleanup.

'Delete emails from server'-UI was already removed from android/ios in june
(see https://github.com/deltachat/deltachat-android/pull/1406)
and the function never exists on desktop at all.

src/events.rs

@@ -119,10 +119,6 @@ pub enum EventType {
     #[strum(props(id = "105"))]
     ImapMessageMoved(String),
 
-    /// Emitted when an IMAP folder was emptied
-    #[strum(props(id = "106"))]
-    ImapFolderEmptied(String),
-
     /// Emitted when an new file in the $BLOBDIR was created
     #[strum(props(id = "150"))]
     NewBlobFile(String),