Deprecate TerminalReporter._tw

[nicoddemus]

For some time there's an alias writer, which is public and part of the API because plugins may need to consume it.

We should deprecate it and remove it.

[nicoddemus]

Fixed in #2804

[nicoddemus]

Because we don't want to expose TerminalWriter further because we plan to eventually replace it by something else, this was reverted in #2945.