Re: [Refactor]Avoid to handle FORCE_NOT_NULL/FORCE_NULL options when COPY TO

ProcessCopyOptions previously rejects force_quote_all for COPY FROM
and copyfrom.c is not even conscious of the option (that is, even no
assertion on it). The two options are rejected for COPY TO by the same
function so it seems like a thinko of the commit. Getting rid of the
code would be good in the view of code coverage and maintenance.

Yeah, ProcessCopyOptions() does have the check for force_notnull and
force_null whether it is using COPY FROM and whether it is in CSV mode.
So the codes in copyto.c processing force_notnull/force_null are

On the otherhand I wonder if it is good that we have assertions on the
option values.