chore: remove mention of Lean.withSeconds (#5481)

There's a comment on `withHeartbeats` that says "See also
Lean.withSeconds", but his definition does not seem to actually exist.
Hence, I've removed the comment.

src/Lean/Util/Heartbeats.lean

@@ -21,7 +21,6 @@ Remember that user facing heartbeats (e.g. as used in `set_option maxHeartbeats`
 differ from the internally tracked heartbeats by a factor of 1000,
 so you need to divide the results here by 1000 before comparing with user facing numbers.
 -/
--- See also `Lean.withSeconds`
 def withHeartbeats [Monad m] [MonadLiftT BaseIO m] (x : m α) : m (α × Nat) := do
   let start ← IO.getNumHeartbeats
   let r ← x