[14.0] report_xlsx: Fix currency format

[mmequignon]

Some xlsx readers doesn't like mixed formats on the same column.
Here we sometimes have the sign on the left, sometimes on the right.
(change introduced in this commit on account-financial-report OCA/account-financial-reporting@f088aa6#diff-8237378723eb85bb5c6d6e16f339b55840b762029442371284fe1ad71d08c1e2
When it happens, we get ### displayed (even though value is ok)
-> No, this isn't because of the column being too narrow.

Among the tested readers, I was able to reproduce on excel (asking to repair the file), onlyoffice (not proposing any fix) and gsheet.

Reverting to a simple number format.

Also, the only report on which this method is open items in account_financial_report, which also displays the currency in a dedicated column. This information is redundant.

[simahawk]

@victoralmau you introduced this change if I'm not mistaken. Could you please have a look?

[victoralmau]

The existing change (actually introduced in 13.0 #647 +1 year ago) is intentional, the idea is to not need to add a currency column and display the value with the currency (as appropriate). I have (and no one has reported it so far) not had (or known) any problems viewing the generated excel files, but I think the right thing to do is to make it compatible and not remove the existing utility.