Introduce a new ERP5Site_resetAccountingTransactionLineGroupingReference
script to reset grouping reference on accounting transaction lines.

This solve a problem with the way we activated on
AccountingTransactionLine_resetGroupingReference the context of the
line, if line was removed from OFS before activity was executed, the
activity was discarded and grouping references on related lines was not
reset.

Also drop the unused `keep_if_valid_group` parameter, it was not making
sense.