fixed: linux internet detection with non-ethernet non-WLAN connections

There is a check for ethernet encapsulation in the Linux network
detection code. This is because we used to provide a network
configuration dialog, and the settings only made sense for devices with
ethernet encapsulation (ethernet, WLAN, etc).

However, the internet connectivity detection relies on the same code,
causing XBMC to disallow the user from entering e.g. the "Video sources"
section if they are connected to the internet via e.g. 3G modem or via
a bluetooth connection.

Fix that by simply removing the check, since we do not provide the IP
configuration facilities anymore.

While this may raise the possibility of false positives a little (if
there are non-internet non-ethernet interfaces), it is better to err on
the side of caution and not prevent the user from using internet sources
in XBMC.