Browser compat: FileStream.Unix.cs referencing not supported API

[buyaa-n]

FlushWriteBufferForWriteByte(), FillReadBufferForReadByte() methods of FileStream.Unix.cs referencing API not supported on browser

Please create an issue for this and link it from here. We have to fix it as the code does not work as-is

Originally posted by @marek-safar in #43363 (comment)

[adamsitnik]

In the new and default strategy for FileStream introduced in #55191 we don't use this API anymore, so I am going to close this issue.