[TASK] Use official TYPO3.Icon repository

The backend icon set has been moved and is now under maintenance of the
TYPO3 GitHub Department. https://github.com/TYPO3/TYPO3.Icons

Resolves: #79404
Releases: master, 7.6
Change-Id: I9c4a78bbf52f681d61e69f2e7245fb1c2cac561f
Reviewed-on: https://review.typo3.org/51377


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>