[!!!][TASK] Remove flashmessage_compatibility.js from core

The flashmessage_compatibility.js is deprecated since TYPO3 7 and is now
removed from core.

The deprecated flashmessage_compatibility.js is loaded at first and takes
place in ``TYPO3.Severity``. The singleton check of
``TYPO3/CMS/Backend/Severity`` detects that ``TYPO3.Severity`` is already
existing and stops the execution. In fact, ``TYPO3/CMS/Backend/Severity``
is never used.

Resolves: #73445
Releases: master
Change-Id: I87982cd9b6880448d430c77bd16ddfbb4bdc7991
Reviewed-on: https://review.typo3.org/46683


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>