Remove code that was needed for <2.10 compatibility

There is still an occurrence mentioning 2.9 compatibility, but I don't know what it does, thus I didn't touch it: https://github.com/scala-ide/scala-ide/blob/master/org.scala-ide.sdt.core/src/scala/tools/eclipse/javaelements/ScalaStructureBuilder.scala#L841

Another occurrence for 2.9 is here, but I don't understand what the comment means: https://github.com/scala-ide/scala-ide/blob/master/org.scala-ide.sdt.core/src/scala/tools/eclipse/javaelements/ScalaJavaMapper.scala#L136