networkd: remove a weird assertion from network_load_one

The assertion was added in dbffab87f1 but I'm not exactly sure
why. Now it just crashes a fuzzer I wrote. I, of course, could fix the fuzzer
so that it would generate names ending in ".network" if anyone would tell
me why the assertion should be kept here.

src/network/networkd-network.c

@@ -181,8 +181,6 @@ static int network_load_one(Manager *manager, const char *filename) {
         if (!d)
                 return -EINVAL;
 
-        assert(streq(d, ".network"));
-
         *d = '\0';
 
         network->required_for_online = true;