[tz] North Vietnam 1970 to 1975 - Asia/Hanoi belongs into main file

[Paul Eggert]

On 6/28/20 12:57 PM, pierreneter wrote:
> # For timestamps in north Vietnam back to 1970 (the tzdb cutoff),
>> # use Asia/Bangkok; see the VN entries in the file zone1970.tab.
> 
> Should we replace that Asia/Bangkok to Asia/Ho_Chi_Minh in the comment?

No, as that would be mishandle timestamps in much of south Vietnam from 1970 to
1975-06-13.

That being said, perhaps the comment could be improved to make things clearer;
suggestions are welcome.