This code is not used by the new fluid simulation system.
Therefore, it does not make sense to keep it around.
We do want to integrate the fluid system with the generic cache
system eventually, but refactoring the system is easier when there
is less dead code that has to be refactored.
Note, I could not remove BKE_ptcache_id_from_smoke entirely yet,
because the depsgraph seems to expect that object that
uses DEG_add_collision_relations also has a pointcache.
That is because it wants to reset the point cache when any of the
other objects changed (this does not work with fluids currently).
Reviewers: sebbas
Differential Revision: https://developer.blender.org/D8987
e5c0d4613a