no more subject prefix for xetex mailing list

[Karl Berry]

Hi - I've removed the "[XeTeX]" prefix on Subject lines from this
mailing list (xetex)'s messages. It is my hope that this will reduce
the ever-increasing flood of dmarc failures
(https://en.wikipedia.org/wiki/DMARC).

This was first noticed with Yahoo, long ago, which did not matter much,
then with icloud etc., which was worse, and now I see it from lots of
other email providers, government institutions, universities, large
companies, etc.  It is hard to discern exactly when the errors happen
(often it's when address x is subscribed, and x forwards to y), but they
are increasingly common.

The List-Id: header identifies the mailing list for mailing list
messages. I know it is less convenient, but that is the Internet today.

I'm going to be doing this on other tug lists too as time goes on; all
of them eventually, I expect.

I am discouraged by the need to kowtow to dmarc's draconian breakage of
standard mailing list usage, but there is no viable alternative. -k