exclude qttranslations from cherry-pick scheme

for the reasons explained in the comments of QTQAINFRA-3883, i want to keep the qttranslations repo in forward merge mode. if there are tangible technical reasons which prevent this, they need to be properly elucidated here.

the only required admin change i'm aware of is telling the sanity bot to complain about the right things again: override the with-pickbot option in qt/qttranslations.git/config.