Remove special Tensorflow installation instructions for M1 Macs

[egpbos]

Hopefully, soon, a new Tensorflow release will have packages for M1 Macs ("arm64" in the package name, see e.g. the tensorflow-addons file list where it is already there now). At that point, we should remove the special installation instructions in the readme about this.

Depending on what the new release supports precisely (Python versions and such), we may need to update the readme to say what versions we support.