All mavlink channels that have streamrates are streaming

#### Issue details
The default streamrate for ADSB is 5; it is zero for every other stream: https://github.com/ardupilot/ardupilot/blob/master/ArduCopter/GCS_Mavlink.cpp#L555

Having this non-zero means that the channel is marked as streaming (https://github.com/ardupilot/ardupilot/blob/master/libraries/GCS_MAVLink/GCS_Param.cpp#L294) and we'll do a lot more work figuring out what we should send.

I suggest we change the default stream rate for this parameter to 0.

The other option is to eliminate the streaming mask, because it's useless as-is.

#### Version
master

#### Platform
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

#### Airframe type
All

#### Hardware type
All of them.

#### Logs
n/a