core/cgroup: drop allow_cache parameter in unit_get_io_accounting()

The name of the parameter is misleading and it does not save us much
work because it is not used during regular unit property queries.
It is only used during unit_log_resources(), and the cgroup is already
dead by that point so it won't be read anyway.

src/core/unit.c

@@ -2385,7 +2385,7 @@ static int unit_log_resources(Unit *u) {
 
                 assert(io_fields[k].journal_field);
 
-                (void) unit_get_io_accounting(u, k, k > 0, &value);
+                (void) unit_get_io_accounting(u, k, &value);
                 if (value == UINT64_MAX)
                         continue;