Remove 'Destructive' prompts (#11631)

While these would match how macOS handles this scenario, they crash on
Catalina, and require mouse clicks to interact.

cc @bennetbo



Release Notes:

- N/A

crates/gpui/src/platform.rs

@@ -719,10 +719,6 @@ pub enum PromptLevel {
 
     /// A prompt that is shown when a critical problem has occurred
     Critical,
-
-    /// A prompt that is shown when asking the user to confirm a potentially destructive action
-    /// (overwriting a file for example)
-    Destructive,
 }
 
 /// The style of the cursor (pointer)