delete unused scala.collection.DebugUtils

[xuwei-k]

No description provided.

[xuwei-k]

Note:
introduce 8d31155#diff-8fef823cccab9d92fc65c64e49f0b096R80
scala-parallel-collections use DebugUtils

$ git rev-parse HEAD
e5eb994763e5ab7c31c540ae59d2bc843a35ffe4
$ git grep DebugUtil
core/src/main/scala/scala/collection/parallel/ParIterableLike.scala:  import scala.collection.DebugUtils._
core/src/main/scala/scala/collection/parallel/mutable/ParFlatHashTable.scala:    import scala.collection.DebugUtils._
core/src/main/scala/scala/collection/parallel/mutable/ParHashSet.scala:  import scala.collection.DebugUtils._

• https://github.com/scala/scala-parallel-collections/blob/e5eb994763/core/src/main/scala/scala/collection/parallel/ParIterableLike.scala#L1493
• https://github.com/scala/scala-parallel-collections/blob/e5eb994763/core/src/main/scala/scala/collection/parallel/mutable/ParFlatHashTable.scala#L29
• https://github.com/scala/scala-parallel-collections/blob/e5eb994763/core/src/main/scala/scala/collection/parallel/mutable/ParHashSet.scala#L92