Remove image attribute from the object returned by map_task

[fjorgemota]

Fixes #5931

Changes proposed in this Pull Request

• Remove the "image" attributes from the tasks returned by the endpoint API related to Sensei Home;

Testing instructions

On a WP installation containing Sensei LMS on this branch:

1. Run curl --user "user:application password" http://your-host/wp-json/sensei-internal/v1/home | jq .tasks;
2. Make sure that there are no image attributes in the output of the command above;

[codecov[bot]]

Codecov Report

Merging #5932 (b2c965e) into feature/new-onboarding-experience (541edc2) will increase coverage by 0.15%. The diff coverage is 73.33%.

@@                           Coverage Diff                           @@
##             feature/new-onboarding-experience    #5932      +/-   ##
=======================================================================
+ Coverage                                43.83%   43.99%   +0.15%     
- Complexity                                9143     9190      +47     
=======================================================================
  Files                                      439      442       +3     
  Lines                                    32521    32702     +181     
  Branches                                   252      252              
=======================================================================
+ Hits                                     14257    14387     +130     
- Misses                                   18078    18129      +51     
  Partials                                   186      186              

Impacted Files	Coverage Δ
...cludes/admin/home/tasks/class-sensei-home-task.php	0.00% <ø> (ø)
...class-sensei-home-task-configure-learning-mode.php	27.27% <ø> (+4.19%)	:arrow_up:
...ask/class-sensei-home-task-create-first-course.php	52.17% <ø> (+4.17%)	:arrow_up:
...e/tasks/task/class-sensei-home-task-setup-site.php	0.00% <ø> (ø)
...sk/class-sensei-home-task-publish-first-course.php	72.41% <72.41%> (ø)
...in/home/tasks/class-sensei-home-tasks-provider.php	100.00% <100.00%> (ø)
includes/admin/class-sensei-home.php	0.00% <0.00%> (-11.87%)	:arrow_down:
includes/admin/class-sensei-admin-notices.php	48.41% <0.00%> (-3.48%)	:arrow_down:
...cludes/rest-api/class-sensei-rest-api-internal.php	67.56% <0.00%> (-1.01%)	:arrow_down:
includes/class-sensei-autoloader.php	14.58% <0.00%> (-0.32%)	:arrow_down:
... and 8 more

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