Cleanup: Remove unused tree element types

These weren't used since ages. Comment out the DNA define (value shouldn't be
reused to avoid compatibility breakage) and remove icon code for the types.

source/blender/editors/space_outliner/outliner_draw.c

@@ -2287,12 +2287,6 @@ TreeElementIcon tree_element_get_icon(TreeStoreElem *tselem, TreeElement *te)
       case TSE_R_LAYER:
         data.icon = ICON_RENDER_RESULT;
         break;
-      case TSE_LINKED_LAMP:
-        data.icon = ICON_LIGHT_DATA;
-        break;
-      case TSE_LINKED_MAT:
-        data.icon = ICON_MATERIAL_DATA;
-        break;
       case TSE_POSEGRP_BASE:
       case TSE_POSEGRP:
         data.icon = ICON_GROUP_BONE;

source/blender/makesdna/DNA_outliner_types.h

@@ -95,9 +95,9 @@ typedef enum eTreeStoreElemType {
   TSE_R_LAYER_BASE = 19,
   TSE_R_LAYER = 20,
   /* TSE_R_PASS          = 21, */ /* UNUSED */
-  TSE_LINKED_MAT = 22,
+  /* TSE_LINKED_MAT = 22, */
   /* NOTE, is used for light group */
-  TSE_LINKED_LAMP = 23,
+  /* TSE_LINKED_LAMP = 23, */
   TSE_POSEGRP_BASE = 24,
   TSE_POSEGRP = 25,
   TSE_SEQUENCE = 26,     /* NO ID */