This adds an operation mode to the Face Set Edit tool which deletes the
geometry of a Face Set by clicking on it.
The operator also checks for the mesh having a single Face Set to avoid
deleting the entire object by accident.
This is also disabled for Multires to avoid modifying the limit surface
without control (it is not an important limitation as base meshes for
multires are usually final, but maybe it can be supported in the future).
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D8938
c15bd1c4e6