[FIX] account: revert 6de2990f and proper fix

Revert "[FIX] account: reset the move reference on bank statement"

This reverts commit 6de2990f.

The previous patch was removing the reference of the move.
However, the reference is useful to reuse the name if the line is reconciled
again.

Instead, remove the constrain. To delete the move, account_cancel is already
needed. It is useful to allow removing statement lines to match the statements
form the bank and the one on Odoo.
In the future, allowing to set the amounts to zero should be added as a way to
correct the lines.

opw-741205