In Go1.0, Writer.Flush used to finish off the current file with zeros
(if it was not already finished) and then write the padding.

Since Go1.1, a regression was made (https://golang.org/cl/5777064) where it was
an error to call Flush if the current file was incomplete. Thus, Flush now only
writes out the final padding bytes, which arguably isn't very useful to anyone.
Since this has been the behavior of Flush for 9 releases of Go (1.1 to 1.9),
we should keep this behavior and just simplify the logic.

We also mark the method as deprecated since it serves no purpose.

Change-Id: I94610d942cb75cad495efd8cf799c1a275a21751
Reviewed-on: https://go-review.googlesource.com/54434
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>