[!!!][TASK] Remove system extension t3skin

The system extension "t3skin" has been slowly trimmed down due to
the native CSS/LESS building of the TYPO3 core into EXT:backend and
EXT:core, and through the IconProvider / IconFactory concepts.

The leftover files, which are mainly related to ExtJS (which
will be removed soon anyway) have been moved to EXT:core for the time
being.

All other images are unused and deleted from the TYPO3 core.

Resolves: #79259
Releases: master
Change-Id: Ic0a0c47999ca57d4e6ea5d7499afc399b307b15f
Reviewed-on: https://review.typo3.org/51252


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>