refactor(template): eliminate manual indexing on certain Nginx variables due to inherent default indexing

[ADD-SP]

Summary

The following ngx vars were added by #11722 for performance:

lua_kong_load_var_index $http_x_kong_request_debug;
lua_kong_load_var_index $http_x_kong_request_debug_token;
lua_kong_load_var_index $http_x_kong_request_debug_log;

And the lua-kong-nginx-module index these by default now: https://github.com/Kong/lua-kong-nginx-module/releases/tag/0.9.0, and we have already bumped it by #12752, so we can remove these explicit indexing now.

Checklist

• [N/A] The Pull Request has tests
• [N/A] A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• [N/A] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

N/A

[team-gateway-bot]

Successfully created cherry-pick PR for master:

• https://github.com/kong/kong-ee/pull/8552