fix: drop no longer supported dialyzer option

apps/emqx/rebar.config

@@ -50,7 +50,7 @@
 ]}.
 
 {dialyzer, [
-    {warnings, [unmatched_returns, error_handling, race_conditions]},
+    {warnings, [unmatched_returns, error_handling]},
     {plt_location, "."},
     {plt_prefix, "emqx_dialyzer"},
     {plt_apps, all_apps},

apps/emqx_resource/rebar.config

@@ -10,10 +10,6 @@
 
 {extra_src_dirs, ["examples"]}.
 
-%% try to override the dialyzer 'race_conditions' defined in the top-level dir,
-%% but it doesn't work
-{dialyzer, [{warnings, [unmatched_returns, error_handling]}]}.
-
 {deps, [
     {jsx, {git, "https://github.com/talentdeficit/jsx", {tag, "v3.1.0"}}},
     {emqx, {path, "../emqx"}}