Remove unused or unnecessary helpers of ScriptPromiseUntyped

* IsObject() and IsNull() are unused
* IsUndefinedOrNull() has a single, DCHECK-only usage
* GetIsolate() is only used where we already have a ScriptState
  from which to get the isolate.

Change-Id: I7d4520890928b6dc2e5b80f61289140a66001b7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401466


Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279881}