[position] Add a doc comment to PieceBitBoards::all_pieces; remove unused empty_squares

position/src/position/piece_sets.rs

@@ -51,14 +51,12 @@ impl PieceBitBoards {
         }
     }
 
+    /// A BitBoard representing all the pieces currently on the board. Other
+    /// engines might refer to this concept as 'occupancy'.
     pub(crate) fn all_pieces(&self) -> &BitBoard {
         self.by_color.all()
     }
 
-    pub(crate) fn empty_squares(&self) -> BitBoard {
-        !self.by_color.all()
-    }
-
     pub(crate) fn all_pieces_of_color(&self, color: Color) -> &BitBoard {
         self.by_color.bitboard(color)
     }