chore(emqx): emqtt as test dependency

apps/emqx/rebar.config

@@ -18,7 +18,6 @@
     , {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v4.0.1"}}} %% todo delete when plugins use hocon
     , {hocon, {git, "https://github.com/emqx/hocon.git", {tag, "0.5.0"}}}
     , {pbkdf2, {git, "https://github.com/emqx/erlang-pbkdf2.git", {branch, "2.0.4"}}}
-    , {emqtt, {git, "https://github.com/emqx/emqtt", {tag, "1.2.3.1"}}}
     , {recon, {git, "https://github.com/ferd/recon", {tag, "2.5.1"}}}
     , {snabbkaffe, {git, "https://github.com/kafka4beam/snabbkaffe.git", {tag, "0.13.0"}}}
     ]}.