Remove unused arguments from split_selfjoin_quals()

Hi,

While reviewing a patch for a self-join elimination bug, I noticed that
split_selfjoin_quals() takes "from" and "to" arguments. These names also
seemed a little inconsistent with the "remove" and "keep" terminology
used by the surrounding SJE code.

Looking further, I found that neither of these arguments is actually
used by the function. The "root" argument is unused as well. I also
checked the history and found that these arguments have been unused
since the SJE code was initially committed.

The attached patch removes these three unused arguments and simplifies
the function interface. There is no functional change.

--
Thanks,
Tender Wang