edit prediction: Don't show and discard completion if toggled off in the buffer (#24927)

Discards an inline completion when it's toggled to off (using, say, a
keyboard shortcut). This matches the behaviour in VS Code and JetBrains,
and I think is a bit more intuitive.

(https://github.com/zed-industries/zed/discussions/24895)

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -1953,8 +1953,13 @@ impl Editor {
         cx: &mut Context<Self>,
     ) {
         self.show_inline_completions_override = show_edit_predictions;
+
+        if let Some(false) = show_edit_predictions {
+            self.discard_inline_completion(false, cx);
+        } else {
             self.refresh_inline_completion(false, true, window, cx);
         }
+    }
 
     fn inline_completions_disabled_in_scope(
         &self,