Revert "sys/kbio.h: support Unicode key codes in vt keymap files"

It has been pointed out, that this change causes ABI breakage for
[GP]IO_DEADKEYMAP. I'll create a review on phabricator.

Since the 8 bit limit on keycodes causes issues for certain keymaps,
a fix should be committed in time to allow a MFC to 13.2.

This reverts commit 1e0853ee84031e4131a0b8cc8737696f199d3d4c.

Reported by: Jessica Clarke