electricity tariff quarter hourly prices: remove global variables

[LKuemmel]

löst den offenen Punkt aus #2801

[tpd-opitz]

Traceback (most recent call last):
  File "/var/www/html/openWB/packages/modules/loadvars.py", line 139, in ep_get_prices
    self.price_value_store.update()
  File "/var/www/html/openWB/packages/modules/common/store/_tariff.py", line 62, in update
    raise FaultState.from_exception(e)
  File "/var/www/html/openWB/packages/modules/common/fault_state.py", line 85, in from_exception
    self.fault_str, self.fault_state = exceptions.get_default_exception_registry().translate_exception(
  File "/var/www/html/openWB/packages/helpermodules/exceptions/registry.py", line 28, in translate_exception
    return "{} {}".format(type(exception), exception.args), FaultStateLevel.ERROR
AttributeError: 'NoneType' object has no attribute 'args'

[tpd-opitz]

Der Timestamp des ersten Eintrages ändert sich nicht
https://forum.openwb.de/viewtopic.php?p=136463#p136463