test: move `maybe_expr` enable into rebar.config

rebar.config

@@ -1,5 +1,7 @@
 %% -*- mode: erlang -*-
-{erl_opts, [debug_info]}.
+
+{erl_opts, [debug_info, {feature, maybe_expr, enable}]}.
+
 {deps, [
     {erlavro, {git, "https://github.com/emqx/erlavro.git", {tag, "2.10.0"}}}
 ]}.

src/emqx_plugrel.erl

@@ -1,9 +1,5 @@
 -module(emqx_plugrel).
 
--feature(maybe_expr, enable).
-
--include_lib("kernel/include/file.hrl").
-
 -export([init/1, do/1, format_error/1]).
 
 -define(METADATA_VSN, <<"0.1.0">>).