cleanup: remove adaptive las N config option

The feature has been replaced so the option no longer applies.

config.js

     disableStats: false,
     disableAudioLevels: false,
     channelLastN: -1, // The default value of the channel attribute last-n.
-    adaptiveLastN: false,
     //disableAdaptiveSimulcast: false,
     enableRecording: false,
     enableWelcomePage: true,