core: remove NM_SETTING_PARAM_INFERRABLE flag from bridge-port.path-cost

In NetworkManager we have a default port path-cost equal to 100.
In the linux kernel the default port cost depends upon the interface
speed: 2 for 10Gb, 4 for 1Gb, 19 for 100Mb and 100 for 10Mb (or when the
interface speed is not available, like current virtio_net driver).
Allow NetworkManager to assume bridge port connections also when the
path-cost differs: this will allow us to assume bridge ports created
outside NetworkManager (e.g. in initrd) that will likely have a different
"cost" value.

(cherry picked from commit cad905fce2)

libnm-core/nm-setting-bridge-port.c

@@ -264,7 +264,6 @@ nm_setting_bridge_port_class_init (NMSettingBridgePortClass *klass)
 		                    0, BR_MAX_PATH_COST, 100,
 		                    G_PARAM_READWRITE |
 		                    G_PARAM_CONSTRUCT |
-		                    NM_SETTING_PARAM_INFERRABLE |
 		                    G_PARAM_STATIC_STRINGS));
 
 	/**