Refactor: remove BKE_<id_type>_copy functions.

Those were only shallow wrappers around `BKE_id_copy`, barely used (even
fully unused in some cases), and we want to get rid of those ID-specific
helpers for the common ID management tasks. Also prevents weird custom
behaviors (like `BKE_object_copy`, who was the only basic ID copy
function to reset user count of the new copy to zero).

Part of 71219.

source/blender/editors/curve/editcurve.c

@@ -1438,7 +1438,7 @@ static int separate_exec(bContext *C, wmOperator *op)
     DEG_relations_tag_update(bmain);
 
     newob = newbase->object;
-    newcu = newob->data = BKE_curve_copy(bmain, oldcu);
+    newcu = newob->data = BKE_id_copy(bmain, &oldcu->id);
     newcu->editnurb = NULL;
     id_us_min(&oldcu->id); /* Because new curve is a copy: reduce user count. */