iflib: remove the unused TX queue state machine

[netchild]

Summary

The previous commit stopped using ift_qstatus and the IFLIB_QUEUE_*
states for the TX watchdog decision, leaving only dead stores.
Remove the field, the states, and all assignments. The byte the
field frees stays behind as explicit padding. No functional change.

MFC after: 1 month
Assisted-by: Claude Code (Fable 5, Opus 5)

[netchild]

Make the padding explicit.