core: layout: drop display_symbol function

The display_label function supersedes the old display_symbol function.
Drop the old one.

src/core/layout.rs

@@ -367,17 +367,6 @@ impl Part {
             l => l,
         }
     }
-
-    pub fn display_symbol(&self, mod_state: &[ModState]) -> Keysym
-    {
-        let mut sym = self.val.0;
-
-        if mod_state[MOD_SHIFT - 1] != ModState::Released {
-            sym = Self::modify_shift(sym);
-        }
-
-        sym
-    }
 }
 
 pub struct Key {