Remove unused use item

`null_ssh_contour` is not used since commit e0a05ba6.

Inst_eddies/set_contours.f90

@@ -17,7 +17,7 @@ contains
 
     use config_m, only: min_amp
     use cont_list_m, only: write_cont_list, hshp_cont_list
-    use derived_types, only: extremum, null_ssh_contour, ssh_contour
+    use derived_types, only: extremum, ssh_contour
     use get_1_outerm_m, only: get_1_outerm
     use set_max_speed_m, only: set_max_speed