Remove redundant static_cast<void*> in examples.

[TobiSchluter]

Not only is it unnecessary because zmq::socket_t automatically converts
to void* via operator void*, it is also misleading as the operator is
called in spite of the appearance that it's "just" a cast.

Now, other comments in zmq.hpp indicate that this is not really the
intent (note the comments near the bool conversion), and I believe the
cleanest fix would be to add a member to zmq::socket_t that returns the
void* needed for polling (.pollable()? I really can't find a good name),
or for zmq::pollitem_t to handle the conversion instead of having this
magical operator. But since this attempts to document what's there, I
think we should document what is there now while avoiding confusing
bits. As it is there is a C++ type zmq::pollitem_t which can be filled
from zmq::socket_t without jumping through hoops, so let's document
that even though one could infer from the man page for zmq_poll that
a static_cast<void*> is needed.

This PR is related to #858

[TobiSchluter]

Well all these failing checks seem to complain about C# in a filename in parts that I didn't touch but maybe I misunderstand?