Obsolete comment in partbounds.c

[Etsuro Fujita]

While reviewing the partitionwise-join patch, I noticed $Subject，ie,
this in create_list_bounds():

                /*
                 * Never put a null into the values array, flag instead for
                 * the code further down below where we construct the actual
                 * relcache struct.
                 */
                if (null_index != -1)
                    elog(ERROR, "found null more than once");
                null_index = i;

"the code further down below where we construct the actual relcache
struct" isn't in the same file anymore by refactoring by commit
b52b7dc25.  How about modifying it like the attached?

Best regards,
Etsuro Fujita

fix-obsolete-comment.patch
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