Removes bad comment from CookieManager

A change that made the setCookie() docs say setCookie rejected
session cookies was introduced here
https://googleplex-android-review.git.corp.google.com
/#/c/166946/3/core/java/android/webkit/CookieManager.java
and looks like it was just a mistaken rephrasing of the original docs.

This change removes that comment from the current docs.

Bug: 14379829
Change-Id: I36ae708f55faf424424ea18f75c89872e7c71312