No longer need engine_task_tags so remove

src/engine.h

@@ -98,16 +98,6 @@ enum engine_step_properties {
 #define engine_default_timesteps_file_name "timesteps"
 #define engine_max_parts_per_ghost 1000
 
-/* MPI task tags. */
-enum engine_task_tags {
-  engine_task_tag_xv = 0,
-  engine_task_tag_rho,
-  engine_task_tag_ti,
-  engine_task_tag_grad,
-  engine_task_tag_grav,
-  engine_tasks_per_tag
-};
-
 /**
  * @brief The rank of the engine as a global variable (for messages).
  */