Remove air-quality since WeatherBIT API has stopped serving free air-quality data

[vatsa287]

• Remove get_airquality_data.py & related files
• Remove airquality subparser

[vatsa287]

Since WeatherBIT has stopped giving API service for air-quality as free and made it as a premium service, cli-weather will no longer will be able to support it as well. So remove all files which uses this feature, will remove this feature by next release through PyPI binary linux build. Main changes to be made:

• Remove the subparser 'airquality; since its no longer required.
• Remove the python file which enables the above subparser i.e get_airquality.
• Remove its dependencies from both get_by_city.py and get_py_postalcode.py

This would be a nice feature enhancement for a first time Pull Request. Will look into incorporate any other provider as airquality API provider.