Remove coeffects from listIdentifiers and listAbbreviations, marking …

…them as impure

Summary: See D67602326 and T211026366.  These functions are incorrectly marked as pure, but pull impure data from sources external to WWW.  This causes issues for codegen, which can become non-deterministic due to the lack of hermeticity of these functions.

Reviewed By: latrekc

Differential Revision: D67602402

fbshipit-source-id: 3ff8fb589051a750ffdd3ec19156ef8ad341a88b

hphp/hack/hhi/stdlib/builtins_datetime.hhi

@@ -395,11 +395,11 @@ class DateTimeZone {
     int $timestamp_begin = PHP_INT_MIN,
     int $timestamp_end = PHP_INT_MAX,
   )[]: HH\FIXME\MISSING_RETURN_TYPE;
-  static public function listAbbreviations()[]: HH\FIXME\MISSING_RETURN_TYPE;
+  static public function listAbbreviations(): HH\FIXME\MISSING_RETURN_TYPE;
   static public function listIdentifiers(
     int $what = 2047,
     string $country = '',
-  )[]: HH\FIXME\MISSING_RETURN_TYPE;
+  ): HH\FIXME\MISSING_RETURN_TYPE;
 }
 
 class DateInterval {