Revert "USe do notation for durationUnits?"

This reverts commit 0f3ac7c.

cedar-lean/Cedar/Spec/Ext/Datetime.lean

@@ -160,15 +160,17 @@ def duration? (i: Int) : Option Duration :=
 def Duration.neg? (d : Duration) : Option Duration :=
   d.val.neg?
 
-def durationUnits? (n: Nat) (suffix: String) : Option Int := do
-  let i ← Int64.ofInt? n
-  match suffix with
-  | "ms" => some i
-  | "s" => some (i * MILLISECONDS_PER_SECOND)
-  | "m" => some (i * MILLISECONDS_PER_MINUTE)
-  | "h" => some (i * MILLISECONDS_PER_HOUR)
-  | "d" => some (i * MILLISECONDS_PER_DAY)
-  | _ => none
+def durationUnits? (n: Nat) (suffix: String) : Option Int :=
+  match Int64.ofInt? n with
+  | none => none
+  | some i =>
+    match suffix with
+    | "ms" => some i
+    | "s" => some (i * MILLISECONDS_PER_SECOND)
+    | "m" => some (i * MILLISECONDS_PER_MINUTE)
+    | "h" => some (i * MILLISECONDS_PER_HOUR)
+    | "d" => some (i * MILLISECONDS_PER_DAY)
+    | _ => none
 
 def unitsToMilliseconds (days hours minutes second milliseconds: Int) : Int :=
   days * MILLISECONDS_PER_DAY +