Remove unused field on Task (#531)

Kelli asked me yesterday what this field is used for and... uhm. Guess
*someone* left it there.

Author: mrnugget

internal/batches/executor/executor.go

@@ -69,8 +69,7 @@ type Task struct {
 	// If Path is "" then this setting has no effect.
 	OnlyFetchWorkspace bool
 
-	Steps   []batches.Step
-	Outputs map[string]interface{}
+	Steps []batches.Step
 
 	// TODO(mrnugget): this should just be a single BatchSpec field instead, if
 	// we can make it work with caching