[TASK] Drop backend.js

The file backend.js contained some basic preparations for the backend
context that are partially not required anymore. The remaining settings
are converted to a global assignment set in BackendController.

The `window.name` property is now hard-coded to be `typo3-backend` for
use with the EXT:adminpanel, which requires some changes in acceptance
tests as `window.name` had an empty value before.
`window.opener` is nulled, otherwise, this might cause issues with
inter-frame communication if the backend was e.g. opened via a link in
an email.

With the remaining assigned being migrated, backend.js serves no purpose
anymore and is dropped now.

Resolves: #99101
Releases: main
Change-Id: I1b92184c1e5063f7a494c62060adfb5b67a2878e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76607


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>