block_copy_do_copy() is static, only used in block_copy_task_entry
with the error_is_read argument set. No need to check for it,
simplify.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200507121129.29760-3-philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>