refactoring: remove unneeded transform in "tube.delete"

All the queue drivers do "transform(2, 1, state.DONE)"
before returning the task from their "delete" method.
Also, changing the status of a task from abstract.lua
is a violation of encapsulation.

Patch changes:
* remove the unnecessary "transform" operation from "tube.delete"
* update description of a driver delete method in documentation

Co-authored-by: Leonid Vasiliev<lvasiliev@tarantool.org>

Author: aleclarson

README.md

@@ -631,7 +631,8 @@ which is passed on to the user (removes the administrative fields)
 Returns a normalized task which represents a tuple in the space
 * `tube:take()` - sets the task state to 'in progress' and returns the task.
 If there are no 'ready' tasks in the queue, returns nil.
-* `tube:delete(task_id)` - deletes a task from the queue
+* `tube:delete(task_id)` - deletes a task from the queue.
+Returns the original task with a state changed to 'done'
 * `tube:release(task_id, opts)` - puts a task back to the queue (in the 'ready'
    state)
 * `tube:bury(task_id)` - buries a task

queue/abstract.lua

@@ -223,8 +223,7 @@ end
 
 function tube.delete(self, id)
     self:peek(id)
-    return self.raw:normalize_task(
-        self.raw:delete(id):transform(2, 1, state.DONE))
+    return self.raw:normalize_task(self.raw:delete(id))
 end
 
 -- drop tube