Drop no-longer used caps.Exists trait

Author: odersky

library/src/scala/caps.scala

@@ -51,12 +51,6 @@ import annotation.{experimental, compileTimeOnly, retainsCap}
    */
   extension (x: Any) def readOnlyCapability: Any = x
 
-  /** A trait that used to allow expressing existential types. Replaced by
-   *  root.Result instances.
-   */
-  @deprecated
-  sealed trait Exists extends Capability
-
   /** This should go into annotations. For now it is here, so that we
    *  can experiment with it quickly between minor releases
    */