Revert KDBUS_HELLO_ACCEPT_MEMFD support (ABI break)

Revert KDBUS_HELLO_ACCEPT_MEMFD support (ABI break)

Memfds are a basic exchange mechanism not supposed to be
optional per connection. A per-bus flag, instead of a
per-connection one, would probably be acceptable,
but its usefulness is questionable at this point.

Broadcasts can contain memfds and we would silently messages
for such connections, which is not the expected behavior.

Receivers just need to make sure to be able to receive messages
with memfd payload, otherwise they are just not fully supporting
the common kdbus interface.

Contracts of not supporting memfds on private buses are fine,
but the general purpose communication will always require
memfds to be supported by all clients.