Remove some uses of set_wml_tag_metatable

(cherry-picked from commit 3939c332b9)

data/multiplayer/eras.lua

@@ -1,5 +1,4 @@
-local helper = wesnoth.require "helper"
-local T = helper.set_wml_tag_metatable {}
+local T = wml.tag
 
 local res = {}
 

data/multiplayer/scenarios/2p_Dark_Forecast.lua

@@ -1,7 +1,7 @@
 
 local helper = wesnoth.require("helper")
 local _ = wesnoth.textdomain 'wesnoth-multiplayer'
-local T = helper.set_wml_tag_metatable {}
+local T = wml.tag
 local on_event = wesnoth.require("on_event")
 
 local random_spawns = {

data/scenario-test.cfg

@@ -1011,8 +1011,7 @@ local types = { "Ancient Lich", "Ancient Wose", "Elvish Avenger" }
 
 local result = wesnoth.synchronize_choice(function()
     -- Human choice
-    local helper = wesnoth.require "helper"
-    local T = helper.set_wml_tag_metatable {}
+    local T = wml.tag
     local _ = wesnoth.textdomain "wesnoth"
     local dialog = {
       T.helptip { id = "helptip_large" },

data/test/scenarios/test_lua_wml_tagnames.cfg

@@ -5,7 +5,7 @@
         name = prestart
         [lua]
             code = <<
-                local T = wesnoth.require("lua/helper.lua").set_wml_tag_metatable {}
+                local T = wml.tag
 
                 local function assert_equal(expect, status)
                     if status ~= expect then