test: remove ct_helpers from apps/emqx rebar.config

apps/emqx/rebar.config

@@ -29,8 +29,8 @@
    {test,
        [{deps,
            [ meck
+           , proper
            , {bbmustache,"1.10.0"}
-           , {emqx_ct_helpers, {git,"https://github.com/emqx/emqx-ct-helpers.git", {tag,"2.1.0"}}}
            , {emqtt, {git, "https://github.com/emqx/emqtt", {tag, "1.4.3"}}}
            ]},
          {extra_src_dirs, [{"test",[recursive]}]}