Don't show alert on already set hostname

[mcspr]

Inspired by the #1030

[xoseperez]

Don't think this is a good idea. The previous behavior was wrong and that was the reason to force the hostname to be RFC compliant.

[mcspr]

I would argue it is the same case as device password - it does not issue an alert when already set password does not match regexp.
Maybe show a warning about possible issue but do not block configuration change? Issue mentions mqttTopic {hostname} usage which will break external topic matching. While technically correct, it breaks already working setup.