Use a passive_t.

This was missed because of the Yodaesque GOD_CHEIBRIADOS == you.religion.
There is a change in behaviour, in that the poison reduction is no longer
effective under penance.

crawl-ref/source/player.cc

@@ -4521,7 +4521,8 @@ void handle_player_poison(int delay)
     // If Cheibriados has slowed your life processes, poison affects you less
     // quickly (you take the same total damage, but spread out over a longer
     // period of time).
-    const double delay_scaling = (GOD_CHEIBRIADOS == you.religion && you.piety >= piety_breakpoint(0)) ? 2.0 / 3.0 : 1.0;
+    const double delay_scaling = have_passive(passive_t::slow_metabolism)
+                               ? 2.0 / 3.0 : 1.0;
 
     const double new_aut = cur_aut - ((double) delay) * delay_scaling;
     const double new_dur = _poison_aut_to_dur(new_aut);