Remove comment that is no longer true

In WT-6568 (this commit)) michael.cahill changed some of the split_parent() code to this:

     *

     * Getting a new split generation implies a full barrier, no additional barrier is needed.

     */ 

    WT_FULL_BARRIER();

    split_gen = __wt_gen(session, WT_GEN_SPLIT);

    parent->pg_intl_split_gen = split_gen;

That is, we include a full barrier immediately after commenting that no barrier is needed. I don't know the details of the change, so I'm not sure what the comment should say or if it should simply be removed.

Note, this same change appears in 3 places in that file.

sue.loverso and keith.smith, you were the other two folks involved in that change, can somebody please fix it?