Error is logged when no env is provided, even though it's optional

[Voxelot]

According to the coding guidelines, we should only use ERROR level if there is a very serious error. https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/logging.md

However, right now we log an error if we can't load the fuel-core settings from an env file even though a user may prefer to use cli args instead. This is a jarring experience for users and we should just remove the logging here.