Drop -Ywarn-unused:privates for 2.12

[majk-p]

When working with an sbt plugin with sbt-tpolecat I came across an issue caused by -Ywarn-unused:privates. It produces false positives in some cases. Combined with fatal warnings this breaks my build for no reason. Run following gist with scala-cli (or latest scala) for reproduction: https://gist.github.com/majk-p/9d6dfffce77688c835ddd148fcc3af77

It's caused by a bug in Scala 2.12 discussed scala/bug#9294 and as stated by the maintainers it won't be fixed.

I'd like to suggest not including this flag for 2.12 sbt modules to avoid confusion