Revert "Add setting to show/hide title bar (#37428)" (#38756)

Closes https://github.com/zed-industries/zed/issues/38547

Release Notes:

- Reverted the ability to show/hide the titlebar. This caused rendering
bugs on
macOS, and we're preparing for the redesign which requires the toolbar
being present.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/workspace/src/workspace.rs

@@ -2040,11 +2040,6 @@ impl Workspace {
         cx.notify();
     }
 
-    pub fn clear_titlebar_item(&mut self, _: &mut Window, cx: &mut Context<Self>) {
-        self.titlebar_item = None;
-        cx.notify();
-    }
-
     pub fn set_prompt_for_new_path(&mut self, prompt: PromptForNewPath) {
         self.on_prompt_for_new_path = Some(prompt)
     }