Remove an obsolete comment in gistinsert()

Hi,

While learning the GIST codes, I find an obsolete comment in gistinsert ().

itup = gistFormTuple(giststate, r,
values, isnull, true /* size is currently
bogus */ );

The comment "/* size is currently bogus */" is weird because it follows a
bool variable.
I tracked the commit history. The original gistFormTuple() prototype is as
below:

extern IndexTuple gistFormTuple(GISTSTATE *giststate,
Relation r, Datum *attdata, int *datumsize, bool
*isnull);

you can see this commit 37c8393 to check that.

After 1f7ef54, the function prototype changed, as below:

extern IndexTuple gistFormTuple(GISTSTATE *giststate,
Relation r, Datum *attdata, bool *isnull, bool
newValues);

As you can see, "int *datumsize" had been removed, but the comments in
gistbuildCallback() and gistinsert() were not removed together.

By the way, after commit 5edb24a, the comments in gistbuildCallback() was
removed.
So I think we can now remove the obsolete comment from gistinsert().

--
Thanks,
Tender Wang