Remove TODO for n/a - min() handles correctly

As @tsalo pointed out in bids-standard#2075 (comment)
there is a test in expression_tests.yaml which ensures that -1 is less than n/a

Author: yarikoptic

src/schema/rules/checks/events.yaml

@@ -54,5 +54,4 @@ NonNegativeOnsets:
     - suffix == "events"
     - extension == ".tsv"
   checks:
-    # TODO: how to handle n/a values?
     - min(columns.onset) < 0