BMesh: remove checks for tessellating 2 sided faces

2 sided faces aren't supported and will cause problems in many areas
of Blender's code.

Removing (implied) support for faces with fewer than 3 sides
means the total number of triangles is known ahead of time.

This simplifies adding support for multi-threading and partial updates
to an existing tessellation - as the face and loop indices can be used
to access the range of triangles associated with a face.

Also correct outdated comments.

source/blender/python/bmesh/bmesh_py_types.c

@@ -1385,7 +1385,6 @@ static PyObject *bpy_bmesh_calc_loop_triangles(BPy_BMElem *self)
   BMesh *bm;
 
   int looptris_tot;
-  int tottri;
   BMLoop *(*looptris)[3];
 
   PyObject *ret;
@@ -1398,10 +1397,10 @@ static PyObject *bpy_bmesh_calc_loop_triangles(BPy_BMElem *self)
   looptris_tot = poly_to_tri_count(bm->totface, bm->totloop);
   looptris = PyMem_MALLOC(sizeof(*looptris) * looptris_tot);
 
-  BM_mesh_calc_tessellation(bm, looptris, &tottri);
+  BM_mesh_calc_tessellation(bm, looptris);
 
-  ret = PyList_New(tottri);
-  for (i = 0; i < tottri; i++) {
+  ret = PyList_New(looptris_tot);
+  for (i = 0; i < looptris_tot; i++) {
     PyList_SET_ITEM(ret, i, BPy_BMLoop_Array_As_Tuple(bm, looptris[i], 3));
   }
 

source/blender/python/mathutils/mathutils_bvhtree.c

@@ -961,8 +961,6 @@ static PyObject *C_BVHTree_FromBMesh(PyObject *UNUSED(cls), PyObject *args, PyOb
 
   /* Get data for tessellation */
   {
-    int tris_len_dummy;
-
     coords_len = (uint)bm->totvert;
     tris_len = (uint)poly_to_tri_count(bm->totface, bm->totloop);
 
@@ -971,8 +969,7 @@ static PyObject *C_BVHTree_FromBMesh(PyObject *UNUSED(cls), PyObject *args, PyOb
 
     looptris = MEM_mallocN(sizeof(*looptris) * (size_t)tris_len, __func__);
 
-    BM_mesh_calc_tessellation(bm, looptris, &tris_len_dummy);
-    BLI_assert(tris_len_dummy == (int)tris_len);
+    BM_mesh_calc_tessellation(bm, looptris);
   }
 
   {