Remove brush-preset: Eraser_Airbrush

Eraser_Airbrush is a duplicate of Eraser_Soft. It does
same thing, and it does less better than selecting
directly Airbrush_Soft and turning it to Eraser.
Also, I never thought of using a real-brush airbrush
to erase something; no-logic. If this is missing for someone,
I'll propose a extra-soft eraser instead of this.