4de60244 added the call to table_finish_bulk_insert to the
CopyMultiInsertBufferCleanup function.  We use a CopyMultiInsertBuffer even
for non-partitioned tables, so having the cleanup do that meant we would
call table_finsh_bulk_insert twice when performing COPY FROM with
a non-partitioned table.

Here we can just remove the direct call in CopyFrom and let
CopyMultiInsertBufferCleanup handle the call instead.