Use shift_remove with IndexMap

This fixes a deprecation warning as the `remove` method had been
deprecated in favor of either `swap_remove` or `shift_remove`.

It seems appropriate to use `shift_remove` here to preserve ordering.

src/lib.rs

@@ -228,7 +228,7 @@ impl PrefixMapping {
     ///
     /// * [`PrefixMapping::add_prefix()`]
     pub fn remove_prefix(&mut self, prefix: &str) {
-        self.mapping.remove(prefix);
+        self.mapping.shift_remove(prefix);
     }
 
     /// Expand a CURIE, returning a complete IRI.