Don't make callers responsible for losestr death

Remove callers' responsibility to deal with possible hero death when
calling losestr.  This is less fragile and error-prone than leaving it
in the caller's hands, but it means that death from the monster spell
'weaken target' no longer goes through done_in_by, and the death reason
is no longer "killed by <monster name>".

src/mcastu.c

@@ -465,11 +465,7 @@ cast_wizard_spell(struct monst *mtmp, int dmg, int spellnum)
             dmg = mtmp->m_lev - 6;
             if (Half_spell_damage)
                 dmg = (dmg + 1) / 2;
-            losestr(rnd(dmg));
-            if (u.uhp < 1)
-                done_in_by(mtmp, DIED);
-            else if (Upolyd && u.mh < 1)
-                rehumanize();
+            losestr(rnd(dmg), (const char *) 0, 0);
         }
         dmg = 0;
         break;