Revert addition of new base trait to ReturnThrowable

The deprecated class NonLocalReturns.ReturnThrowable acquired a new base type
`caps.Control` in 3.8. That was a mistake. Later versions of the capture checker
reject that inheritance since `ReturnThrowable` is an exception and therefore
must be a pure class, whereas inheriting `caps.Control` would automatically
add a capture set to that class.

It's too late to revert for 3.8, but we should revert for 3.8.1. It's technically
a backwards incompatible change, but I would be extremely surprised if anyone has
used ReturnThrowable as a caps.Control in the 3.8.0 window.

library/src/scala/util/control/NonLocalReturns.scala

@@ -24,7 +24,7 @@ import scala.compiletime.uninitialized
 @deprecated("Use scala.util.boundary instead", "3.3")
 object NonLocalReturns {
   @deprecated("Use scala.util.boundary.Break instead", "3.3")
-  class ReturnThrowable[T] extends ControlThrowable, caps.Control {
+  class ReturnThrowable[T] extends ControlThrowable {
     private var myResult: T = uninitialized
     def throwReturn(result: T): Nothing = {
       myResult = result

project/MiMaFilters.scala

@@ -19,6 +19,7 @@ object MiMaFilters {
 
       // Breaking changes since last reference version
       Build.mimaPreviousDottyVersion -> Seq(
+        ProblemFilters.exclude[MissingTypesProblem]("scala.util.control.NonLocalReturns$ReturnThrowable")
       )
     )
   }