Revert "Particle system: Move runtime data to runtime field"

This reverts commit 36faf739a7.

Somewhat annoying but this change had some unforeseen consequences,
which lead to an actual bug.

Since this change was not sufficient to get original report fixed
is easier to simply revert for now.

Fixes T65842: Hair disappears when clicking on particle system name

source/blender/modifiers/intern/MOD_explode.c

@@ -1125,7 +1125,7 @@ static Mesh *applyModifier(ModifierData *md, const ModifierEvalContext *ctx, Mes
     if (psys->part == NULL || psys->particles == NULL) {
       return mesh;
     }
-    if (BKE_particle_modifier_mesh_final_get(psmd) == NULL) {
+    if (psmd->mesh_final == NULL) {
       return mesh;
     }