[BUGFIX] Remove autoload information for t3skin extension

There is no Classes folder anymore in t3skin, hence no autoloading can take place.
Remove the reference in the composer.json

Resolves: #70123
Releases: master
Change-Id: I9b9dd3a79b9b40980f7732daa5ad98aa812e1ae1
Reviewed-on: http://review.typo3.org/43579


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>