common/poller.c: remove the obsolete defer_{read,write}() API

These two functions were used in one of my earliest attempts
to use a libevent-based core for Neubot.

They were not used anymore in LibNeubot (and I wanted to delete
them), so I feel obliged to remove them from Libight.

No breakages or new warnings by rebuilding all on MacOSX.
common/poller.c: remove the obsolete resolve() API

This API was used to experiment with Neubot, but it was scheduled
for removal from LibNeubot.

So, let's just remove it.

After this change, the Poller is small enough that the next step
can be to rewrite it using C++11.

As before, I verified that everything still builds smoothly.