Move out-of-scope Arabian peninsula zones to 'backzone'.

* asia (Asia/Bahrain): Now links to Asia/Qatar.
(Asia/Kuwait, Asia/Aden): Now links to Asia/Riyadh.
(Asia/Muscat): Now links to Asia/Dubai.
* backzone: Move the old data here.
* checktab.awk (tztab): Remove special cases for these zones.
* NEWS: Document the above.