api/execution: remove ProcessID from rpc calls

Now that the shim handles all container's processes the system pid is
sufficient.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

execution/events.go

@@ -10,7 +10,7 @@ type ContainerEvent struct {
 
 type ContainerExitEvent struct {
 	ContainerEvent
-	PID        string
+	PID        uint32
 	StatusCode uint32
 }