Remove nearly-unused SizeOfIptrData macro.

Past refactorings have removed all but one reference to SizeOfIptrData
(and that one place was in a pretty noncritical spot).  Since nobody's
complained, it seems probable that there are no supported compilers
that don't think sizeof(ItemPointerData) is 6.  If there are, we're
wasting MAXALIGN per heap tuple anyway, so it's rather silly to worry
about whether we can shave space in places like WAL records.

Pavan Deolasee

Discussion: <CABOikdOOawDda4hwLOT6zdA6MFfPLu3Z2YBZkX0JdayNS6JOeQ@mail.gmail.com>