remove outdated comment

tests/testthat/test-ecxsys.R

@@ -171,7 +171,7 @@ test_that("the curves have not changed", {
 test_that("the returned fn works the same way as internally", {
     # I don't know why it would fail but it doesn't hurt to test it.
     curves <- mod$curves
-    curves$concentration_for_plots <- NULL  # remove column "use_for_plotting"
+    curves$concentration_for_plots <- NULL
     expect_identical(curves, predict_ecxsys(mod, curves$concentration))
 })