Remove LineBucket.addFeature

[tmcw]

Looks like this method adds indirection without encapsulation

https://github.com/mapbox/mapbox-gl-js/blob/77acaedd0069165c0916dc9a899f2184b78cd4b7/js/data/line_bucket.js#L13-30

[tmcw]

Same with FillBucket: I don't see any reason for addFeature to be a function. https://github.com/mapbox/mapbox-gl-js/blob/77acaedd0069165c0916dc9a899f2184b78cd4b7/js/data/fill_bucket.js#L12-24

[jfirebaugh]

Tests are using addFeature and the vector-tile API does not make it easy to remove right now. Maybe if mapbox/vector-tile-js#34 lands.