Mesh and mask smoothing were combined into a single funciton and task
with a smooth_mask variable to control the data that functions should
modify. This comes from before 2.81, when the smooth brush had the
implementation of neighbor connecteivity for mesh, bmesh and multires,
in order to share all that code.

With the Sculpt API now is trivial to implement a smooth function, so it
makes sense to have separate functions to affect different data which can
be tweaked individually to create more advanced brushes.