[Keyboard] YMD96 refactor (#5472)

[Keyboard] YMD96 refactor (#5472)

* Remove JJ50 data from YMD96

JJ50 was actually added as its own keyboard when this was added in #2546. It should have been taken out then, but wasn't.

* Update ymd96.h

- use #pragma once include guard
- remove redundant file includes

* Update LAYOUT_iso macro to K<row><col> notation

* Update LAYOUT_custom macro to K<row><col> notation

* Update LAYOUT_default macro to K<row><col> notation

* Refactor default keymap

* Rename readme file to lowercase

* Rename layers enum and default layer

- renamed layers enum to layer_names
  - proposed by fauxpark in Issue 5977, and I like the idea
    - https://github.com/qmk/qmk_firmware/issues/5977#issuecomment-495924338
- renamed the base layer to _DEFAULT
  - I think it looks nicer.