Remove: silencer-lib

Silencer is no longer necessary. 2.11 support should also be removed from libraries if it hasn't been already. > NOTE: Scala 2.13.2 and 2.12.13 introduced https://github.com/scala/scala/pull/8373. This means that unless you're still cross compiling for Scala 2.11, this plugin is obsolete, and you should use [@nowarn](https://www.scala-lang.org/api/current/scala/annotation/nowarn.html). https://github.com/ghik/silencer