Revert "[FIX] Hooks now correctly check if counterside implemented hook"

Not all hooks were corrected and it seems it is not clear which part of the code check for null. (It is once done in the OSN/ISN and furthmore in the hooks itself.)

This reverts commit 86d312667a59b71295c26a65314bf2fe7ff1c826.

Change-Id: I4644a2e3a98f8334c7a6f3b1f8c90cf7ce4f3dbd
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1715
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Tested-by: Jenkins CI