Remove obsolete link to ancient site.

A better link would be gwtproject/gwt#3034, which in turn points to gwtproject/gwt#3319.

But the purpose of the link was to explain why our `ImmutableEnumSet` was _not_ serializable, and it became serializable back in cl/13204390 (as belatedly documented in cl/13216866). I should have removed the comment at that point.

And of course now we've been ripping out GWT serialization entirely, so that that's yet another reason to eliminate this comment.

RELNOTES=n/a
PiperOrigin-RevId: 700411695

android/guava/src/com/google/common/collect/Sets.java

@@ -101,7 +101,6 @@ public boolean retainAll(Collection<?> c) {
    * @param otherElements the rest of the elements the set should contain
    * @return an immutable set containing those elements, minus duplicates
    */
-  // http://code.google.com/p/google-web-toolkit/issues/detail?id=3028
   @GwtCompatible(serializable = true)
   public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(
       E anElement, E... otherElements) {
@@ -118,7 +117,6 @@ public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(
    * @param elements the elements, all of the same {@code enum} type, that the set should contain
    * @return an immutable set containing those elements, minus duplicates
    */
-  // http://code.google.com/p/google-web-toolkit/issues/detail?id=3028
   @GwtCompatible(serializable = true)
   public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(Iterable<E> elements) {
     if (elements instanceof ImmutableEnumSet) {

guava/src/com/google/common/collect/Sets.java

@@ -103,7 +103,6 @@ public boolean retainAll(Collection<?> c) {
    * @param otherElements the rest of the elements the set should contain
    * @return an immutable set containing those elements, minus duplicates
    */
-  // http://code.google.com/p/google-web-toolkit/issues/detail?id=3028
   @GwtCompatible(serializable = true)
   public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(
       E anElement, E... otherElements) {
@@ -120,7 +119,6 @@ public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(
    * @param elements the elements, all of the same {@code enum} type, that the set should contain
    * @return an immutable set containing those elements, minus duplicates
    */
-  // http://code.google.com/p/google-web-toolkit/issues/detail?id=3028
   @GwtCompatible(serializable = true)
   public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(Iterable<E> elements) {
     if (elements instanceof ImmutableEnumSet) {