Remove temporary namespace extension

Now included with TypeScript 5.1

utils/supportedCanonicalTimezones.ts

@@ -1,22 +1,3 @@
-/**
- * Intl.supportedValuesOf is not shipped until Typescript 5.1
- * Temporary workaround is from
- * https://github.com/microsoft/TypeScript/issues/49231#issuecomment-1137251612
- *
- * TODO: When Typescript 5.1 is released with Deno (check with `deno --version`), remove this namespace extension.
- */
-declare namespace Intl {
-  type Key =
-    | "calendar"
-    | "collation"
-    | "currency"
-    | "numberingSystem"
-    | "timeZone"
-    | "unit";
-
-  function supportedValuesOf(input: Key): string[];
-}
-
 /**
  * Get all canonical timezones that are supported by the browser or runtime,
  * or an empty array if the list is not available.