remove unused app_compilers app env setting (#1903)

src/rebar.app.src

@@ -40,8 +40,8 @@
                      {pkg, rebar_pkg_resource},
                      {hg, rebar_hg_resource}]},
 
-        {app_compilers, [{erl, rebar_erlc_compiler}]},
-        {compilers, [rebar_compiler_xrl, rebar_compiler_yrl, rebar_compiler_mib, rebar_compiler_erl]},
+        {compilers, [rebar_compiler_xrl, rebar_compiler_yrl,
+                     rebar_compiler_mib, rebar_compiler_erl]},
 
         {providers, [rebar_prv_app_discovery,
                      rebar_prv_as,