aio / timers: Remove main_loop_timerlist

Now we have timerlistgroups implemented and main_loop_tlg, we
no longer need the concept of a default timer list associated
with each clock. Remove it and simplify initialisation of
clocks and timer lists.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>