[backport] Delete color manager from AbstractScalaScanner and subtypes

(cherry picked from commit 0b26673d9948d5160d5b51f1aacbdda66fcf43b6)

I had to slightly adapt the set of changes, as we are only interested in
cherr-picking the deletion of ``colorManager`` from AbstractScalaScanner and
subtypes. Below follows the list of changes wrt the original commit:

* Use ``SingleTokenScanner`` in place of ``ScalaCommentScanner`` 
* Removed ScalaCommentScanner.scala from the set of cherry-picked changes

Conflicts:

    org.scala-ide.sdt.core/src/scala/tools/eclipse/ScalaSourceViewerConfiguration.scala
    org.scala-ide.sdt.core/src/scala/tools/eclipse/lexical/ScalaCommentScanner.scala