remove a cast from assignment to same type

src/light.c

@@ -542,7 +542,7 @@ light_sources_sanity_check(void)
         if (!ls->id.a_monst)
             panic("insane light source: no id!");
         if (ls->type == LS_OBJECT) {
-            otmp = (struct obj *) ls->id.a_obj;
+            otmp = ls->id.a_obj;
             auint = otmp->o_id;
             if (find_oid(auint) != otmp)
                 panic("insane light source: can't find obj #%u!", auint);