Keymap: Add german layout for redox keyboard (#3695)

Keymap: Add german layout for redox keyboard (#3695)

*     Add german layout for redox keyboard

    A german layout for the redox keyboard which tries
    to stay close to the default english redox layout
    while acommodating the umlauts at their usual position.

* Fix keycodes in german redox keymap

The keycode for Y was wrong.

* Minor changes

* Remove #ifdef include guard by #pragma once
* Remove unnecessary include from rules.mk

* Review changes

Remove unnecessary code fragments that
were pointed out in the review
of PR #3695 by @drashna.