Remove scala.Product0.

The addition of that trait is not essential to Scala 3. While it
can be argued that it should exist, there is nothing special in
Scala 3 compared to Scala 2 that makes it necessary. In the spirit
of reusing the Scala 2.13 standard library with as few deviations
as possible, we therefore remove it.

It could be added back when we revise the standard library on its
own.

Author: sjrd

library/src/scala/Product0.scala

@@ -248,9 +248,16 @@ object Tuple {
 /** A tuple of 0 elements */
 type EmptyTuple = EmptyTuple.type
 
-/** A tuple of 0 elements; the canonical representation of a [[scala.Product0]]. */
-object EmptyTuple extends Tuple with Product0 {
+/** A tuple of 0 elements. */
+object EmptyTuple extends Tuple {
+  override def productArity: Int = 0
+
+  @throws(classOf[IndexOutOfBoundsException])
+  override def productElement(n: Int): Any =
+    throw new IndexOutOfBoundsException(n.toString())
+
   def canEqual(that: Any): Boolean = this == that
+
   override def toString(): String = "()"
 }