itimers: strip unused bits from struct itimer and struct itimers

[kib]

Summary

Also:

itimers_alloc: no need to initialize its_timers array

struct itimers is allocated with M_ZERO, setting all members to NULL
is tautological.

This seems to be unused definitions, going back to David Xu times and work. At least, kernel builds and boots multiuser with the patch.

[imp]

I agree: those elements have been unused for a long time.

[mav]

I haven't touched that code for years, so I have no idea what are those unused definitions and won't miss them.

[markj]

sys/sys/timers.h

59 ↗

(On Diff #100619)

It looks like it_timerid is write-only too. Maybe I'm missing something.

[kib]

Remove it_timerid.