This change reverts the following CLs:
	CL/18274: handle mtime in NTFS/UNIX/ExtendedTS extra fields
	CL/30811: only use Extended Timestamp on non-zero MS-DOS timestamps

We are reverting support for extended timestamps since the support was not
not complete. CL/18274 added full support for reading extended timestamp fields
and minimal support for writing them. CL/18274 is incomplete because it made
no changes to the FileHeader struct, so timezone information was lost when
reading and/or writing.

While CL/18274 was a step in the right direction, we should provide full
support for high precision timestamps in both the reader and writer.
This will probably require that we add a new field of type time.Time.
The complete fix is too involved to add in the time remaining for Go 1.8
and will be completed in Go 1.9.

Updates #10242
Updates #17403
Updates #18359
Fixes #18378

Change-Id: Icf6d028047f69379f7979a29bfcb319a02f4783e
Reviewed-on: https://go-review.googlesource.com/34651
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>