[iface] Drop redundant device dump in kernel configure()

wgctrl.ConfigureDevice already returns an error when the interface is
missing, so the preceding wg.Device() existence check is redundant. That
check dumps the entire device (all peers) on every configure() call,
making it O(peers) per call and turning bulk peer insertion into
O(peers^2): inserting N peers one by one re-parsed the whole growing peer
list N times. Removing it keeps each peer write constant-time regardless
of how many peers are already configured.

client/iface/configurer/kernel_unix.go

@@ -246,12 +246,6 @@ func (c *KernelConfigurer) configure(config wgtypes.Config) error {
 		}
 	}()
 
-	// validate if device with name exists
-	_, err = wg.Device(c.deviceName)
-	if err != nil {
-		return err
-	}
-
 	return wg.ConfigureDevice(c.deviceName, config)
 }