Simplify China's time zones from five to two.

* NEWS, asia: Document this.
* asia (Asia/Harbin, Asia/Chongqing, Asia/Kashgar): Remove.
These are now links in 'backward'.
(Asia/Shanghai): Change pre-standard-time offset from 8:05:57 to
8:05:43.  Change transition to standard time from 1928 to 1901.
(Asia/Urumqi): Remove 1980 transition to UTC+8.
* backward (Asia/Harbin, Asia/Chongqing, Asia/Kashgar): New links.
(Asia/Chungking): Avoid link-to-link.
* zone.tab (Asia/Harbin, Asia/Chongqing, Asia/Kashgar): Remove.