logind,machined: various smaller cleanups

Use mfree() where we can.

Drop unnecessary {}.

Drop unnecessary variable declarations.

Cast syscall invocations where explicitly don't care for the return
value to (void).

Reword a comment.

src/machine/machine.c

@@ -231,11 +231,11 @@ static void machine_unlink(Machine *m) {
                 char *sl;
 
                 sl = strjoina("/run/systemd/machines/unit:", m->unit);
-                unlink(sl);
+                (void) unlink(sl);
         }
 
         if (m->state_file)
-                unlink(m->state_file);
+                (void) unlink(m->state_file);
 }
 
 int machine_load(Machine *m) {

src/machine/machined-dbus.c

@@ -1124,8 +1124,7 @@ int match_job_removed(sd_bus_message *message, void *userdata, sd_bus_error *err
                 return 0;
 
         if (streq_ptr(path, machine->scope_job)) {
-                free(machine->scope_job);
-                machine->scope_job = NULL;
+                machine->scope_job = mfree(machine->scope_job);
 
                 if (machine->started) {
                         if (streq(result, "done"))