lnwire: remove String() method from FeatureVector
lnwire: remove String() method from FeatureVector

This commit removes the String() method from the FeatureVector as it
can produce confusing output when revving a feature vector and not
knowing the strip mapping ahead of time.

AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd