Ensure disable_ai is properly respected (#34941)

Quick follow up to #34896 which ensures that the Agent Panel cannot be
caught by actions like `workspace: toggle left dock` when `disable_ai`
is set to true.

Also removes a method that was introduced but unused in the workspace
because `first_enabled_panel_idx` already covers all cases this method
could be useful for.

Release Notes:

- N/A

crates/workspace/src/dock.rs

@@ -374,12 +374,6 @@ impl Dock {
             })
     }
 
-    pub fn first_enabled_panel_idx_excluding(&self, exclude_name: &str, cx: &App) -> Option<usize> {
-        self.panel_entries.iter().position(|entry| {
-            entry.panel.persistent_name() != exclude_name && entry.panel.enabled(cx)
-        })
-    }
-
     fn active_panel_entry(&self) -> Option<&PanelEntry> {
         self.active_panel_index
             .and_then(|index| self.panel_entries.get(index))