Remove checks for quotes on ConnectivityCheckUri

While it is true that NetworkManager doesn't sanitize this property on its end, any working set-up will need an already valid URI.

I've come to the conclusion that the quotes I found in my system's config were self inflicted, since the ubuntu package that fills this in (network-manager-config-connectivity-ubuntu) is and always has been ok:

[connectivity]
uri=http://connectivity-check.ubuntu.com./

This also removes a call to the Qt6.8-only QString::slice()