chore: delete hocon from emqx_prometheus application deps

apps/emqx_prometheus/rebar.config

@@ -2,9 +2,7 @@
 
 {deps, [
     {emqx, {path, "../emqx"}},
-    %% FIXME: tag this as v3.1.3
-    {prometheus, {git, "https://github.com/deadtrickster/prometheus.erl", {tag, "v4.8.1"}}},
-    {hocon, {git, "https://github.com/emqx/hocon.git", {tag, "0.29.0"}}}
+    {prometheus, {git, "https://github.com/deadtrickster/prometheus.erl", {tag, "v4.8.1"}}}
 ]}.
 
 {edoc_opts, [{preprocess, true}]}.