promql: Remove falsified comment from test

The test in question actually worked fine even before #16569. The
finding reported in the comment has turned out to be caused by
something else.

Signed-off-by: beorn7 <[email protected]>

Author: beorn7

promql/promqltest/testdata/functions.test

@@ -1060,11 +1060,6 @@ eval instant at 55s avg_over_time(metric8[1m])
 eval instant at 55s avg_over_time(metric9[1m])
   {} 3.225714285714286e+219
 
-# Interestingly, before PR #16569, this test failed with a result of
-# 3.225714285714286e+219, so the incremental calculation combined with
-# Kahan summation (in the improved way as introduced by PR #16569)
-# seems to be more accurate than the simple mean calculation with
-# Kahan summation.
 eval instant at 55s avg_over_time(metric10[1m])
   {} 2.5088888888888888e+219