Revert complete poller subsystem

Revert complete poller subsystem

It seems to have a bad effect on performance that various attempts at
improvement just don't seem to be able fix, so this patch just gets rid
of the whole thing.

This patch reverts the following commits:

 264bde12 "Introduce a threaded DPC into the receiver code"
 129ad516 "Stop using a threaded DPC in the poller"
 5932938b "Don't bump the receiver event counter if the poller is going to retry"
 16002b8f "poller: fix event channels when backends do not support multi-queue"
 bc722edd "Don't use KTIMERs in receive path"
 dfaa68cc "Don't affinitize timer DPC"
 eac9a95a "Move the Receiver and Transmitter event and DPC processing..."
 40be5c12 "Add the boilerplate for a new Poller sub-system"

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>