Remove the compiler plugin from this repo in favor /JetBrains/kotlin

Now the compiler plugin is a part of the Kotlin repo: https://github.com/JetBrains/kotlin/tree/master/plugins/kotlin-dataframe it can safely be removed from the project here.

We might need some migration here, to inform our users where to report issues with the library (here) and with the compiler plugin (YouTrack?).
Also, it might be worth investigating whether we can automate compatibility checks between our master branch and the compiler plugin

Start by locating this repository's references to the compiler plugin now hosted at JetBrains/kotlin/plugins/kotlin-dataframe. Determine what migration guidance users need for library and compiler-plugin issues, and investigate whether compatibility checks between this repository's master branch and the plugin are feasible. Done means the obsolete plugin is removed and the agreed migration or compatibility work is documented or implemented.

Written by the indexing model from the issue text.