[BUGFIX] Avoid registering extension icons in IconRegistry

Extensions usually have an individual icon that can be loaded
from Resources/Public/Icons/Extension.*. For some system extensions
IconRegistry automatically registers icons available in the file
system. This leads to the fact that the "Extension.*"
icon of some extensions is registered multiple times.

Resolves: #83979
Releases: master
Change-Id: I1d2d02a5bbadd33d8d6abeefff0c2071193dbdf0
Reviewed-on: https://review.typo3.org/55828


Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>