Remove TextLoader.Read(string), promote IDataReader<MSS> extension

[TomFinley]

Pursuant to #1090 and #1041 there was agreement that we should be able to just pass a string into a data reader that reads from streams, rather than have to muck about with the creation of this IMultiStreamSource object.

As it happens, this one request was done in two separate ways:

1. ML Context to create them all #1252 where this instance method on TextLoader was created:

machinelearning/src/Microsoft.ML.Data/DataLoadSave/Text/TextLoader.cs

Line 1340 in cb37c7e

public IDataView Read(string path) => Read(new MultiFileSource(path));

2. Extension on IDataReader<IMultiStreamSource>, and DataReader<IMultiStreamSource, TShape> to read from one or several file paths, rather than requiring constructing an IMultiStreamSource #1281 where an extension method on top of IDataReader<IMultiStreamSource> was introduced.

Clearly we don't need both, and the second being more flexible and more broadly applicable was the one that should be checked in, so let us remove the class specific method introduced in #1252. The extension method should however be moved to a namespace where it is more likely to be picked up (probably Microsoft.ML) since right now it's a bit buried and generally people would never see this extension method.