Remove unused argument to the `_async_startup` method

[pdfowler]

While trying to configure the integration, I was using the YAML configuration reloading functionality to select DUALMODE_GENERIC while trying different configurations in my main configuration.yaml file. Whenever I tried to do the targeted reload, I was met with a non-functional climate card and the following in my error log:

Error Logs

Logger: homeassistant.components.climate
Source: custom_components/dualmode_generic/climate.py:401 
Integration: Climate (documentation, issues) 
First occurred: 9:41:30 PM (2 occurrences) 
Last logged: 9:41:30 PM

Error adding entities for domain climate with platform dualmode_generic
Error while setting up dualmode_generic platform for climate
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 382, in async_add_entities
    await asyncio.gather(*tasks)
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 587, in _async_add_entity
    await entity.add_to_platform_finish()
  File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 681, in add_to_platform_finish
    await self.async_added_to_hass()
  File "/config/custom_components/dualmode_generic/climate.py", line 401, in async_added_to_hass
    _async_startup()
TypeError: _async_startup() missing 1 required positional argument: 'event'

Can't say I'm terribly familiar with python, but I can read an error log and can see that the variable isn't being used. By changing the signature from def _async_startup(event): to def _async_startup():, the error went away and I have a new awesome and fully functional dual mode climate control 😁

[zacs]

Appreciate the fix @pdfowler !