Fix stale TODO in BufferedFileStreamStrategy (#117302)

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

@@ -1026,15 +1026,12 @@ private void EnsureBufferAllocated()
             if (_buffer is null)
             {
                 AllocateBuffer();
-            }
-        }
 
-        // TODO https://github.com/dotnet/roslyn/issues/47896: should be local function in EnsureBufferAllocated above.
-        [MemberNotNull(nameof(_buffer))]
-        [MethodImpl(MethodImplOptions.NoInlining)]
-        private void AllocateBuffer()
-        {
-            Interlocked.CompareExchange(ref _buffer, GC.AllocateUninitializedArray<byte>(_bufferSize), null);
+                [MemberNotNull(nameof(_buffer))]
+                [MethodImpl(MethodImplOptions.NoInlining)]
+                void AllocateBuffer() =>
+                    Interlocked.CompareExchange(ref _buffer, GC.AllocateUninitializedArray<byte>(_bufferSize), null);
+            }
         }
 
         [Conditional("DEBUG")]