Remove RGBDS install instructions

[avivace]

The setup article should only provide pointers and not duplicate install instructions from RGBDS official (and maintained) guidelines:

https://gbdev.io/gb-asm-tutorial/part1/setup.html

[Les-Wet]

To my eyes, it seems like the tutorial already directs to the official RGBDS install instructions. The only duplicate instructions I am seeing are potentially the Windows instructions?

[avivace]

@Les-Wet yes, that's correct. This information is present (and explained with much broader context) in the official instructions we link to, so we should remove them from here