remove Options trees

src/main/scala/leon/plugin/CodeExtraction.scala

src/main/scala/leon/purescala/PrettyPrinter.scala

src/main/scala/leon/purescala/ScalaPrinter.scala

src/main/scala/leon/purescala/Trees.scala

src/main/scala/leon/purescala/TypeTrees.scala

src/main/scala/leon/solvers/RandomSolver.scala