Remove poll/select and eventfd stubs

Remove poll/select and eventfd stubs

This commit removes the stubs for functions provided by the
new posix-socket and posix-event libraries.

Signed-off-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #23