Silence unused-nowarn in meta-build

[adpi2]

Fixes #6398

It would be better to have scala/scala#9491 in 2.12 but we don't have it yet.

[adpi2]

Testing locally:

sbt
[info] [launcher] getting org.scala-sbt sbt 1.5.0-SNAPSHOT  (this may take some time)...
[info] welcome to sbt 1.5.0-SNAPSHOT (Ubuntu Java 11.0.10)
[info] loading global plugins from /home/piquerez/.sbt/1.0/plugins
[info] loading settings for project mima-build from plugins.sbt ...
[info] loading project definition from /home/piquerez/lightbend/mima/project
[info] compiling 1 Scala source to /home/piquerez/lightbend/mima/project/target/scala-2.12/sbt-1.0/classes ...
/home/piquerez/lightbend/mima/build.sbt:32: warning: method in in trait ScopingSetting is deprecated (since 1.5.0): `in` is deprecated; migrate to slash syntax - https://www.scala-sbt.org/1.x/docs/Migrating-from-sbt-013x.html#slash
  skip in publish := true,
       ^
/home/piquerez/lightbend/mima/build.sbt:59: warning: method in in trait ScopingSetting is deprecated (since 1.5.0): `in` is deprecated; migrate to slash syntax - https://www.scala-sbt.org/1.x/docs/Migrating-from-sbt-013x.html#slash
  scriptedDependencies := Def.task(()).dependsOn(publishLocal, publishLocal in core).value,
                                                                            ^
/home/piquerez/lightbend/mima/build.sbt:83: warning: method in in trait ScopingSetting is deprecated (since 1.5.0): `in` is deprecated; migrate to slash syntax - https://www.scala-sbt.org/1.x/docs/Migrating-from-sbt-013x.html#slash
    skip in publish := true,
         ^
[info] loading settings for project root from build.sbt ...
[info] set current project to mima (in build file:/home/piquerez/lightbend/mima/)
[info] sbt server started at local:///home/piquerez/.sbt/1.0/server/b07d754fc19c6e72ee47/sock
[info] started sbt server
sbt:mima>

[eed3si9n]

Thanks!