Remove unused Snake::intersects

This method turned out not to be very useful, since we almost always
want to determine the Snake that's 'at fault' for colliding with
another, which we do by checking whose snake's head is colliding.

Author: fwcd

src/model/snake.rs

@@ -47,12 +47,6 @@ impl Snake {
         self.field_set().len() < self.fields.len()
     }
 
-    pub fn intersects(&self, other: &Snake) -> bool {
-        let own_fields = self.field_set();
-        let other_fields = other.field_set();
-        !own_fields.is_disjoint(&other_fields)
-    }
-
     pub fn contains(&self, pos: Pos<i32>) -> bool {
         self.fields.contains(&pos)
     }