remove unnecessary RateLimiterHelper

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/FixedWindowRateLimiter.cs

@@ -24,7 +24,7 @@ public sealed class FixedWindowRateLimiter : ReplenishingRateLimiter
 
         /// <summary>
         /// Function to calculate elapsed time from a given tick value.
-        /// Defaults to <see cref="RateLimiterHelper.GetElapsedTime(long?)"/>.
+        /// Defaults to the <see cref="RateLimiterHelper.GetElapsedTime(long?)"/> which returns <see langword="null"/> when the timestamp is <see langword="null"/>, and returns <c>Stopwatch.GetElapsedTime(long)</c> when not null.
         /// In tests, this field can be reassigned via reflection to inject custom time behavior without modifying the public API.
         /// </summary>
         private readonly Func<long?, TimeSpan?> _getElapsedTime = RateLimiterHelper.GetElapsedTime;
@@ -301,7 +301,7 @@ private void ReplenishInternal(long nowTicks)
                     return;
                 }
 
-                if (RateLimiterHelper.GetElapsedTime(_lastReplenishmentTick, nowTicks) < _options.Window && !_options.AutoReplenishment)
+                if (Stopwatch.GetElapsedTime(_lastReplenishmentTick, nowTicks) < _options.Window && !_options.AutoReplenishment)
                 {
                     return;
                 }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/RateLimiterHelper.cs

@@ -16,10 +16,5 @@ internal static class RateLimiterHelper
 
             return Stopwatch.GetElapsedTime(startTimestamp.Value);
         }
-
-        public static TimeSpan GetElapsedTime(long startTimestamp, long endTimestamp)
-        {
-            return Stopwatch.GetElapsedTime(startTimestamp, endTimestamp);
-        }
     }
 }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/SlidingWindowRateLimiter.cs

@@ -293,7 +293,7 @@ private void ReplenishInternal(long nowTicks)
                     return;
                 }
 
-                if (RateLimiterHelper.GetElapsedTime(_lastReplenishmentTick, nowTicks) < ReplenishmentPeriod && !_options.AutoReplenishment)
+                if (Stopwatch.GetElapsedTime(_lastReplenishmentTick, nowTicks) < ReplenishmentPeriod && !_options.AutoReplenishment)
                 {
                     return;
                 }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/TokenBucketRateLimiter.cs

@@ -308,7 +308,7 @@ private void ReplenishInternal(long nowTicks)
                 }
                 else
                 {
-                    add = _fillRate * RateLimiterHelper.GetElapsedTime(_lastReplenishmentTick, nowTicks).Ticks;
+                    add = _fillRate * Stopwatch.GetElapsedTime(_lastReplenishmentTick, nowTicks).Ticks;
                 }
 
                 _tokenCount = Math.Min(_options.TokenLimit, _tokenCount + add);