socket: don't set interface for sent packets

socket: don't set interface for sent packets

With asymmetric routing (e.g. with BGP) it may not be possible to
respond to a request using the same interface. In such case, setting the
interface index in IP*_PKTINFO* causes the packet to be silently dropped
by the kernel.

Until we can predict if sending with the specified interface will
succeed, or provide only a hint, don't set the interface and leave it
to the kernel to select an interface.

This reverts commit 5fc7674e366f ("ntp: set interface index in
IP*_PKTINFO when responding").

Reported-by: Arkadiusz Miśkiewicz <arekm@maven.pl>