AddCommandSystemTest: remove outdated comment

The bug mentioned in the comment refers to this comment that was in
AddressBook.java:

    // TODO: the tags master list will be updated even though the below line fails.
    // This can cause the tags master list to have additional tags that are not tagged to any person
    // in the person list.

which was added in 1eb8b13 (teach classes in Model to execute edit,
2016-12-13).

However, the bug was fixed in d51894a (AddressBook: remove master tag
list, 2018-03-29).

So, this comment is not relevant any more. Let's remove it.

No other changes will need to be done to the test case itself: any tag
will be suitable for this test case, since HOON himself does not have
any tags.

src/test/java/systemtests/AddCommandSystemTest.java

@@ -133,9 +133,6 @@ public void add() {
         assertCommandFailure(command, AddCommand.MESSAGE_DUPLICATE_PERSON);
 
         /* Case: add a duplicate person except with different tags -> rejected */
-        // "friends" is an existing tag used in the default model, see TypicalPersons#ALICE
-        // This test will fail if a new tag that is not in the model is used, see the bug documented in
-        // AddressBook#addPerson(Person)
         command = PersonUtil.getAddCommand(HOON) + " " + PREFIX_TAG.getPrefix() + "friends";
         assertCommandFailure(command, AddCommand.MESSAGE_DUPLICATE_PERSON);