[Keyboard] QWERTYYdox refactor (#6446)

[Keyboard] QWERTYYdox refactor (#6446)

* Delete null key

`__` key in keymap.c doesn't actually exist on the physical hardware.
Removed key from keymap.c and removed its argument from the layout macro.

* Delete unused keycode aliases

* Replace layer index definitions with an enum

* Replace redundant numpad keycodes with native aliases

* Use native layer change keycodes instead of aliases

* Visually align the keycodes

It makes the keymap pretty.

* Correct Configurator layout data

* Clean up header files

- convert to pragma once include guard
- remove redundant definitions
- remove commented code blocks

* Delete LAYOUT_kc macro

Was copied from ergotravel; not valid for this keyboard.

* Consolidate rev1 rules.mk settings to keyboard level

Previous codebase enabled Backlight at keyboard level then disabled it at revision level.

* Delete unused rules

* Consolidate config.h settings from keymap level to keyboard level

* Modernize keyboard's config.h file

Aligns the keyboard-level config.h file more closely with the current QMK template for AVR keyboards.