Remove redundant conditional using in SpanPointers

The conditional using for System.Buffers is already handled by
GlobalUsings.cs which imports the correct namespace (BCL or
vendored) based on the TFM.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andrewlock

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/AWS/Shared/SpanPointers.cs

@@ -13,12 +13,6 @@
 using Datadog.Trace.ClrProfiler.AutoInstrumentation.AWS.DynamoDb;
 using Datadog.Trace.Util;
 
-#if NETCOREAPP3_1_OR_GREATER
-using System.Buffers;
-#else
-using Datadog.Trace.VendoredMicrosoftCode.System.Buffers;
-#endif
-
 namespace Datadog.Trace.ClrProfiler.AutoInstrumentation.AWS.Shared;
 
 /// <summary>

tracer/src/Datadog.Trace/Util/RandomIdGenerator.cs

@@ -27,6 +27,7 @@
 
 using System;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace Datadog.Trace.Util;