Remove extra string cast from map (chapel-lang#22844)

Removes the potential for a deprecation warning when accessing a map of
a non-primitive key type.

The `Map` module's `KeyNotFoundError` uses `string.format` to create
error message for keys of any type. However, one of map's `this`
assessors was casting the key to string when creating a
`KeyNotFoundError`, which can create a deprecation warning as of
chapel-lang#22068. This PR removes that
cast.

- [x] paratest

[ trivial - not reviewed ]

modules/standard/Map.chpl

@@ -401,7 +401,7 @@ module Map {
 
       var (_, slot) = table.findAvailableSlot(k);
       if !table.isSlotFull(slot) {
-        throw new KeyNotFoundError(k:string);
+        throw new KeyNotFoundError(k);
       }
       ref result = table.table[slot].val;
       return result;