core: drop "override" flag when building transactions

Now that we don't have RequiresOverridable= and RequisiteOverridable=
dependencies anymore, we can get rid of tracking the "override" boolean
for jobs in the job engine, as it serves no purpose anymore.

While we are at it, fix some error messages we print when invoking
functions that take the override parameter.

src/core/timer.c

@@ -511,8 +511,7 @@ static void timer_enter_running(Timer *t) {
         if (unit_stop_pending(UNIT(t)))
                 return;
 
-        r = manager_add_job(UNIT(t)->manager, JOB_START, UNIT_TRIGGER(UNIT(t)),
-                            JOB_REPLACE, true, &error, NULL);
+        r = manager_add_job(UNIT(t)->manager, JOB_START, UNIT_TRIGGER(UNIT(t)), JOB_REPLACE, &error, NULL);
         if (r < 0)
                 goto fail;