[tz] Reason for removal of several TZ

[Derick Rethans]

On Sun, 3 Dec 2017, Paul Eggert wrote:

> At any rate, now that I've pretty much finished the job we should
> document the alphabetic abbreviations used. To give that a go I installed the
> attached patch into the development version on GitHub.

I think you're missing "IST" for "Irish Standard Time", which as far as 
I know is Ireland's legal definition of time (and equivalent with BST), 
with GMT being their "odd one out": 
https://en.wikipedia.org/wiki/Time_in_Ireland / 
http://www.irishstatutebook.ie/eli/1968/act/23/enacted/en/print

cheers,
Derick