kernel: Remove unused timeout_q from z_kernel

[stephanosio]

This commit removes the timeout_q from the struct z_kernel since it
is no longer used.

Note that the new kernel timeout implementation introduced in the
commit 987c0e5 uses timeout_list
global variable in place of it.

Signed-off-by: Stephanos Ioannidis [email protected]

[onkwon]

What about instead removing timeout_list static variable in kernel/timeout.c keeping the list(spinlock as well) in _kernel.timeout_q. I think, this helps to maintain kernel data structure easier.