feat: remove non-sticker heuristics and force_sticker()

This causes problems for Delta Chat Desktop at
<https://github.com/deltachat/deltachat-desktop/pull/6278>
even though the logic was originally introduced for iOS.
If the problem remains on iOS,
heuristics can be added into iOS UI.

src/message.rs

@@ -795,12 +795,6 @@ impl Message {
         self.viewtype
     }
 
-    /// Forces the message to **keep** [Viewtype::Sticker]
-    /// e.g the message will not be converted to a [Viewtype::Image].
-    pub fn force_sticker(&mut self) {
-        self.param.set_int(Param::ForceSticker, 1);
-    }
-
     /// Returns the state of the message.
     pub fn get_state(&self) -> MessageState {
         self.state
@@ -2322,8 +2316,6 @@ pub enum Viewtype {
     Gif = 21,
 
     /// Message containing a sticker, similar to image.
-    /// NB: When sending, the message viewtype may be changed to `Image` by some heuristics like
-    /// checking for transparent pixels. Use `Message::force_sticker()` to disable them.
     ///
     /// If possible, the ui should display the image without borders in a transparent way.
     /// A click on a sticker will offer to install the sticker set in some future.