Remove references to parallel collections in Vector.scala

This makes it build with 2.13.x.

I'll re-run the benchmarks that are already published using the previous version.

Author: lrytz

corpus/vector/fb04376/Vector.scala

@@ -14,7 +14,6 @@ import scala.annotation.unchecked.uncheckedVariance
 import scala.compat.Platform
 import scala.collection.generic._
 import scala.collection.mutable.Builder
-import scala.collection.parallel.immutable.ParVector
 
 /** Companion object to the Vector class
  */
@@ -66,7 +65,6 @@ extends AbstractSeq[A]
    with IndexedSeqLike[A, Vector[A]]
    with VectorPointer[A @uncheckedVariance]
    with Serializable
-   with CustomParallelizable[A, ParVector[A]]
 { self =>
 
 override def companion: GenericCompanion[Vector] = Vector
@@ -80,8 +78,6 @@ override def companion: GenericCompanion[Vector] = Vector
 
   def length = endIndex - startIndex
 
-  override def par = new ParVector(this)
-
   override def toVector: Vector[A] = this
 
   override def lengthCompare(len: Int): Int = length - len