Remove D_u_m_m_y.scala from the stdlib

[nicolasstucki]

We need to remove the scalaShadowing/D_u_m_m_y.scala.

[nicolasstucki]

The current reference compiler expects scalaShadowing. We need to first remove this requirement, then re-bootstrap the compiler, and finally remove D_u_m_m_y.scala.

[nicolasstucki]

It is enough to move to old sources to library/src-non-bootstrapped. We can delete them after we re-bootstrap on M3.