history: show value in XEC units

[PiRK]

There was still a hardcoded decimal_point=8 in the code. This caused the history command line to show values/fees/amounts in MegaXEC instead of XEC.

Before:

$./ElectrumABC-5.1.2-9-gdedc670cb-x86_64.AppImage history
[
    {
        "balance": "0.175",
        "confirmations": 6,
        "date": "2022-02-02 02:02",
        "fee": "0.00000807",
        "height": 777777,
        "label": "",
        "timestamp": 1655555555,
        "txid": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
        "value": "+0.075"
    },
    {
        "balance": "0.1",
        "confirmations": 6,
        "date": "2022-02-02 02:02",
        "fee": "0.00000452",
        "height": 777777,
        "label": "",
        "timestamp": 1655555555,
        "txid": "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy",
        "value": "+0.1"
    }
]