Remove material.ambient, no need for a secondary surface color just for ambient light

Per discussion here: https://github.com/mrdoob/three.js/pull/5741#issuecomment-67893202

Now it should be mentioned that sometimes ```material.ambient``` is considered to be a per-material incoming non-directional light intensity (sometimes called ```material.selfIllumination```, but not to be confused with ```material.emissive```) rather than as a surface color specifically for ambient light as it is treated now  If we wanted to go that route, we would modify the shading equation and leave ```material.ambient``` around, and it would be additive with ambientLightColor, both of which would then be modulated by the ```material.color```.

I am okay with either route.