async_wrap/timers: remove Add/RemoveAsyncListener

async_wrap/timers: remove Add/RemoveAsyncListener

The ability to add/remove an AsyncListener to an object after its
creation was an artifact of trying to get AL working with the domain
module. Now that is no longer necessary and other features are going to
be implemented that would be affected by this functionality. So the code
will be removed for now to simplify the implementation process.

In the future this code will likely be reintroduced, but after some
other more important matters have been addressed.

None of this functionality was documented, as is was meant specifically
for domain specific implementation work arounds.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>