[TASK] Move adminpanel.js to EXT: frontend

Admin panel is displayed on frontend when enabled. Editors can
move this panel via drag+drop based on adminpanel.js.
This file is only used in EXT: frontend and should remain there.

Change-Id: Id0733f6a199487946a312f787c30d8126275ee6b
Resolves: #47943
Releases: 6.2
Reviewed-on: https://review.typo3.org/20518
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe