Remove Choose having mutable, optional implementation

src/main/scala/leon/codegen/CodeGeneration.scala

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/purescala/Expressions.scala

src/main/scala/leon/purescala/Extractors.scala

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

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

src/main/scala/leon/purescala/TypeOps.scala

src/main/scala/leon/repair/Repairman.scala

...main/scala/leon/solvers/templates/TemplateGenerator.scala

src/main/scala/leon/synthesis/rules/CEGISLike.scala

src/main/scala/leon/xlang/ImperativeCodeElimination.scala