Switch to ZWJ approach for Tai Noi conjuncts

No more converting PHINTHU-joined sequences to ad hoc conjunct code
points. Instead, simply allow ZWJ inputting to join characters.

* src/lx-tn-kbd.c (tn_keycode_map):
  - Change qwerty's '&' key to ZWJ (U+200D).
* src/lx-tn-im-table.h
  - Remove macros for ad hoc conjunct code points.
* src/lx-tn-im-table.c (lx_tn_im_conversion):
  - Remove codes that generate ad hoc conjuncts.