Fix critical error noticed by Massimo: copy.c used to have a

Fix critical error noticed by Massimo: copy.c used to have a
special hack to ensure it would close its output file even after failure
due to elog(ERROR) partway through the copy.  This is now unnecessary
because fd.c takes care of cleaning up open files at transaction abort;
worse, after fd.c closed the file copy.c would try to do so *again* at
the start of the next COPY command.  This would result in havoc in most
implementations of stdio library.