Removed unused "collider grid" struct from hair volumetrics.

This is a leftover from previous approach of hair collisions (with
insufficient results). The hair volumetrics actually implements
"collision" with solid objects as well, but uses a Neumann boundary
condition on the main grid for this purpose.

source/blender/physics/intern/hair_volume.c

@@ -93,13 +93,6 @@ typedef struct HairVertexGrid {
 	struct SimDebugData *debug_data;
 } HairVertexGrid;
 
-typedef struct HairColliderGrid {
-	HairGridVert *verts;
-	int res;
-	float gmin[3], gmax[3];
-	float scale[3];
-} HairColliderGrid;
-
 #define HAIR_GRID_INDEX_AXIS(vec, res, gmin, scale, axis) ( min_ii( max_ii( (int)((vec[axis] - gmin[axis]) / scale[axis]), 0), res-2 ) )
 
 BLI_INLINE int hair_grid_offset(const float vec[3], int res, const float gmin[3], const float scale[3])

source/blender/physics/intern/implicit.h

@@ -166,7 +166,6 @@ bool BPH_mass_spring_force_spring_goal(struct Implicit_Data *data, int i, const
 /* ======== Hair Volumetric Forces ======== */
 
 struct HairVertexGrid;
-struct HairColliderGrid;
 
 struct Object;
 struct VoxelData;