Remove extraneous call to GetMemory (#4419)

frameworks/CSharp/aspnetcore/PlatformBenchmarks/Utilities/BufferWriter.cs

@@ -72,8 +72,7 @@ namespace PlatformBenchmarks
                 Commit();
             }
 
-            _output.GetMemory(count);
-            _span = _output.GetSpan();
+            _span = _output.GetSpan(count);
         }
 
         private void WriteMultiBuffer(ReadOnlySpan<byte> source)