Remove saving grace

This feature was no longer fulfilling its intended purpose: the
issue is that when players are aware of it, instead of creating
interesting stories, it becomes a resource the player can rely on.

Alternative mitigation for some "unfair" early-game deaths has been
added (such as the warning shot for early-game attack wands and the
ability of iron shoes to protect against certain early traps), and
the warning shot in particular can create interesting gameplay
moments and stories of its own, especially when saving grace is
*not* present (discovering that a monster has a wand of fire and
can oneshot you is an interesting emergency situation, but less
scary if you know that its first attempt to oneshot you will fail).

The feature was also proving quite hard to code correctly, because
there are numerous cases that it "obviously" shouldn't affect
(most notably beheading due to Vorpal Blade, but also things like
purple worm digestion) which would need to be special-cased, and
because there are grey areas like wand zap bounces (which might or
might not be an intentional attempt by the player to hit themself
with the bounce) and damage from traps. Removing it saves the need
to work out, for every new damage source, whether and when saving
grace should interact with it.

Breaks save compatibility.

include/you.h

@@ -428,7 +428,6 @@ struct you {
     Bitfield(uinvulnerable, 1); /* you're invulnerable (praying) */
     Bitfield(uburied, 1);       /* you're buried */
     Bitfield(uedibility, 1);    /* blessed food detect; sense unsafe food */
-    Bitfield(usaving_grace, 1); /* prevents death once */
     Bitfield(uhandedness, 1); /* There is no advantage for either handedness.
                                  The distinction is only for flavor variation
                                  and for use in messages. */