S65-X and S65-Plus updates and refactoring (#4780)

S65-X and S65-Plus updates and refactoring (#4780)

* S65-X: remove S65-Plus support

The original QMK codebase for the Sentraq S65-X actually supported both the S65-X and the S65-Plus. In the interim, the S65-Plus has been broken off into its own directory.

This commit removes support for the S65-Plus from the `keyboards/s65_x/` directory, as that code has been superseded by the code in the S65-Plus directory (`keyboards/s65_plus/`).

- deleted S65-Plus layout macros from s65_x.h and info.json
- deleted s65plus keymap directory
- removed references to the unused column pins
- removed the two unused columns for the switch matrices
- renamed switch K300 in LAYOUT_ansi to K301 (reflects matrix position)
- renamed switch K214 in LAYOUT_iso to K114 (reflects matrix position)

* S65-X: keymap refactor

- all keymaps now use #include QMK_KEYBOARD_H
- default and iso keymaps refactored for readability
- deleted redundant KC_TRNS and KC_NO keycode definitions from smt keymap

* S65-X: readme update

- updated Hardware Availability link
- updated Docs links

* S65-Plus: add LAYOUT_iso data

Adds LAYOUT_iso macro to s65_plus.h and info.json, and an ISO layout version of the default keymap.

* S65-Plus: refactor default keymap

- refactor for alignment/readability
- removed fn_actions code block
- add empty process_record_user block

* S65-Plus: readme update

- Hardware Availability link is now a hyperlink
- updated Docs links

* S65-X: enable 65_ansi and 65_iso community layouts

Thi commit allows the Sentraq S65-X to use the 65_ansi and 65_iso community layouts.

- LAYOUT_ansi renamed to LAYOUT_65_ansi
- LAYOUT_iso renamed to LAYOUT_65_iso
- added LAYOUTS rule to rules.mk