remove unnecessary null-check on hmon_hitmon_poison()

`obj` here is always non-null, otherwise it leads segv at earlier code.

src/uhitm.c

@@ -1453,7 +1453,7 @@ hmon_hitmon_poison(
         You_feel("like an evil coward for using a poisoned weapon.");
         adjalign(-1);
     }
-    if (obj && !rn2(nopoison)) {
+    if (!rn2(nopoison)) {
         /* remove poison now in case obj ends up in a bones file */
         obj->opoisoned = FALSE;
         /* defer "obj is no longer poisoned" until after hit message */