whisper: only use the node id as a p2p id, not for sending messages (#16102)

This is in preparation for the switch to libp2p: the ID generated
will be from a private key created with the help of libp2p's crypto
library, while Whisper will still use Go's default crypto libraries
for encrypting its messages. This change removes a conflict.

It shouldn't have any impact as the person receiving emails is
the user, not the node.