[BUGFIX] Faulty and superfluous code in import/export module

There is faulty and superfluous code in import/export module:
* ModuleFunctionController does not work at all, this points
  back to issue #22921 during TYPO3 4.4 development
* ImportExportTask fetches thumbnail files from a (faulty)
  storage that are not used at all

Since the required class "mod_user_task" has been removed in
TYPO3 4.4, the module function component was broken since then.
Thus, it will be removed completely with this change.

Fixes: #53555
Releases: 6.2, 6.1, 6.0
Change-Id: I1cbdd967dc47ac2fcd256e7eefc756278822ad84
Reviewed-on: https://review.typo3.org/25302
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn