ENH: Remove VolumeRenderingReplacements fixed point raycaster override
Created by: jcfr
This specialized implementation was first introduced in r4673 (ENH: Integrated VTK ray cast classes, started with modifications for incremental update) by copying the VTK implementation of the FixedPoint volume caster.
Since then, the VTK implementation has matured a lot and fixes can easily be contributed upstream, this commit removes the Slicer specific implementation.