remove publishTo

https://github.com/xerial/sbt-sonatype/blob/0.2.1/src/main/scala/xerial/sbt/Sonatype.scala#L58-L60
https://github.com/xerial/sbt-sonatype/blob/0.2.1/src/main/scala/xerial/sbt/Sonatype.scala#L151-L157