[TASK] Cleanup ext:opendocs

* Remove old file from before namespacing
* Fix imagepath in CSS
* Strict comparison for TYPO3_MODE check

Change-Id: I5d6242f4032d83627284f793e9e79ee13d6de8ae
Resolves: #49550
Releases: 6.2
Reviewed-on: https://review.typo3.org/21772
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind