Revert "Add support for new rebar3 output env option (#10018)"

This option will be enabled on Elixir v1.12 instead.

This reverts commit fc9f769.

Author: josevalim

lib/mix/lib/mix/tasks/deps.compile.ex

@@ -197,14 +197,8 @@ defmodule Mix.Tasks.Deps.Compile do
     config_path = Path.join(dep_path, "mix.rebar.config")
     lib_path = Path.join(config[:env_path], "lib/*/ebin")
 
-    # REBAR_BARE_COMPILER_OUTPUT_DIR is only honored by rebar3 >= 3.14
-    env = [
-      {"REBAR_CONFIG", config_path},
-      {"TERM", "dumb"},
-      {"REBAR_BARE_COMPILER_OUTPUT_DIR", dep_path}
-    ]
-
-    cmd = "#{rebar_cmd(dep)} bare compile --paths #{lib_path}"
+    env = [{"REBAR_CONFIG", config_path}, {"TERM", "dumb"}]
+    cmd = "#{rebar_cmd(dep)} bare compile --paths=#{inspect(lib_path)}"
 
     File.mkdir_p!(dep_path)
     File.write!(config_path, rebar_config(dep))