accounting: reset grouping reference in a "context free" script
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.