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

[Alois Treindl]

(Corrected subject line)

The backzone file contains the zone Asia/Hanoi

It should go into the main file for Asia.

There exists no doubt that in all of what was North Vietnam between at least 1970 and 1975 the timezone was 7h east, and not 8h east as in South Vietnam.

There may be unclear borders, but most of North Vietnam, including its capital, was not unclear at all.

I find it a discrimination of former North Vietnam to not have it represented correctly, at least for the post-1970 period.

What Paul Eggert writes in the comments is a poor excuse.
From Paul Eggert (2019-02-19):                                                                             
#                                                                                                            
# The Ho Chi Minh entry suffices for most purposes as it agrees with all of                                  
# Vietnam since 1975-06-13.  Presumably clocks often changed in south Vietnam                                
# in the early 1970s as locations changed hands during the war; however the                                  
# details are unknown and would likely be too voluminous for this database.                                  
#                                                                                                            
# For timestamps in north Vietnam back to 1970 (the tzdb cutoff),                                            
# use Asia/Bangkok; see the VN entries in the file zone1970.tab.                                             
# For timestamps before 1970, see Asia/Hanoi in the file 'backzone'.