Simplify output_table __index metamethod

From Lua 5.3 docs: "Despite the name, the metamethod for this event can
be either a function or a table. [...] If it is a table, the final
result is the result of indexing this table with key."

nselib/stdnse.lua

@@ -959,9 +959,7 @@ function output_table ()
       end
       rawset(t, k, v)
     end,
-    __index = function (_, k)
-      return t[k]
-    end,
+    __index = t,
     __pairs = function (_)
       return coroutine.wrap(iterator)
     end,