Simplified the functionality of eraser mode

It now no longer changes size by itself. This is more of the nuclear
option than anything, as a lot of brushes DO need to have their size
changed to work well as an eraser, but the previous code was so
user-hostile and counter-intuitive that it was best to entirely remove
it.
Tidy some dead code and unused vars

Removed some dead code and unused vars after the removal of the eraser mode
radius ratio stuff.