fix: incorrect outstanding on non-pos invoice with write_off_account (backport #43316)

[mergify]

Issue

write_off_account is auto set based on the POS Profile when user marks the Invoice as POS - 'Is Pos' checbox.

When write_off_account is set, invoice's Outstanding Amount is not updated. But, if you set and unset 'Is Pos' checkbox, write_off_account is still set, due to which the Outstanding Amount field is not updated even though it is a normal invoice.

This is more pronounced in Sales Return (Credit Note), as the outstanding amount is not even updated in draft status due to which it always shows '0', even if 'Update Outstanding for Self' is enabled.

Fix

If Invoice is not POS, write_off_account will be cleared.

Internal Ref: 20369

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This is an automatic backport of pull request #43316 done by Mergify.