MINOR: init/threads: remove the useless tids[] array

MINOR: init/threads: remove the useless tids[] array

It's still obscure how we managed to initialize an array of integers
with values always equal to the index, just to retrieve the value
from an opaque pointer to the index instead of directly using it! I
suspect it's a leftover from the very early threading experiments.

This commit gets rid of this and simply passes the thread ID as the
argument to run_thread_poll_loop(), thus significantly simplifying the
few call places and removing the need to allocate then free an array
of identity.