Remove currently unused slicing models

As discussed in issue #533 master isn't the right place for these
models, yet. This commit removes these models and makes sure there are
no tables present for them (that might have been added by accident).
Volume Tracing: Icon
Volume Tracing: Proof of concept
Volume Tracing: Server side python
Volume Tracing: Geometry is correctly passed between py and js
Volume Tracing: Trace exterior points pushed to db
Volume Tracing: Trace tool pushes, pulls from db correctly
Volume Tracing: JSTree icons
Volume Tracing: Uses ontology (somewhat incorrectly), tracing window widget added
Volume Tracing: Support holes in db, django
Volume Tracing: Basic functionality works
Volume Tracing: Integrated volume tracing with navigator
Volume Tracing: Move brush slider to Area Segment window
Volume Tracing: Respond to key presses
Volume Tracing: Shift + Mousewheel to change brush size
Volume Tracing: Opacity for traces and canvasLayer
Volume Tracing: View Property propagation, buggy probably due to svg in fabricjs
Volume Tracing: Handle opacity and color explicitly rather than svg
Volume Tracing: Close CMWindow on destroy()
Volume Tracing: dynamic PNG icons for trace instances