Summary:
Before this adding Surface type of force field and removing
this field would leave Surface modifier alive in the stack.
This might be really misleading and annoying.
Now removing force field will ensure no modifiers needed for
it are remained in the stack.
This also fixes missing notifier to redraw modifier stack
when changing force field type.
Reviewers: brecht, campbellbarton
Reviewed By: brecht
Differential Revision: http://developer.blender.org/D13