Remove SNR and NOISE

Since many years drivers don't report noise values for each received packet, so
stop pretending it's possible to use noise values or reliably calculate a SNR
(signal/noise ratio) per packet.

It is possible to get noise values via nl80211 (survey data) but these can't
be exactly related to received packets. I may add the netlink method later, but
for now just remove all noise related code (except I keep it in the output file
and the network packets for backwards compatibility).
Version 4.1