microkernel: deprecate task IRQs

This mechanism does not add enough value to the kernel to be worth
maintaining it. Drivers that need deferred processing of interrupts
can simply define their own task and have the interrupt handler
release an event that the task waits on.

The API is marked as deprecated and it is removed from unit test
coverage as well as the documentation.

Change-Id: Ib87b91cb41e9b6d7fdf0dc62b240a531b6a8889f
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>