Remove play sbt-plugin

[mkurz]

Was added in #158 however I do not understand where it's needed. Maybe I just don't get it, or it is not needed anymore meanwhile? Let's see.

[mkurz]

@Mergifyio backport 6.0.x

[mergify[bot]]

backport 6.0.x

✅ Backports have been created

* [#456 [6.0.x] Remove play sbt-plugin (backport #453) by @mkurz](https://github.com/playframework/scalatestplus-play/pull/456) has been created for branch `6.0.x`