Revert the change to make IMultiplexedStream disposable

[bentoi]

The intent of making IMultiplexedStream disposable was to await on SlicStream tasks and to dispose the Quic stream when the reading and writing sides were completed.

For Slic: awaiting the sending of the stream reset/stop sending/unidirecitonal stream release frame is not needed since these tasks don't throw exceptions other than unexpected exceptions.

For Quic: it doesn't work because ReadsClosed completes to soon and because aborting the stream from the writes closed continuation sporadically cause the peer reading side to complete successfully, see;

• QuicStream.ReadsClosed completes before EOS is consumed dotnet/runtime#79818
• QuicStream.WritesClosed continuation ran before the Quic reset frame is sent? dotnet/runtime#79911