Cleanup: rename BM_mesh_{validate=>is_valid}

The term validate is often used to make the data valid
(e.g. BKE_mesh_validate) where is this only checks the mesh is valid.

source/blender/python/bmesh/bmesh_py_types.cc

@@ -4387,7 +4387,7 @@ int bpy_bm_generic_valid_check(BPy_BMGeneric *self)
  * function where the actual error will be caused by
  * the previous action. */
 #if 0
-    if (BM_mesh_validate(self->bm) == false) {
+    if (BM_mesh_is_valid(self->bm) == false) {
       PyErr_Format(
           PyExc_ReferenceError, "BMesh used by %.200s has become invalid", Py_TYPE(self)->tp_name);
       return -1;