add #removeTask(identifier, [skipConfig])

This adds the ability to remove a registered task from the gruntfile by specifying its identifier.

This will also remove the tasks configuration (with toggle) and its references in other tasks.

While this seemed handy when I started writing this I noticed that when done correctly this would also mean removing the task from all other tasks' configuration (which seems more or less impossible/too much work to me).

In case you still think this feature is a good idea one could at least think about automatic removal from

```javascript
grunt.task.run(['tasToBeRemoved', 'otherTask']);
```