ar71xx: fix syntax error in /lib/ar71xx.sh

Remove all comments in ubnt_xm_board_detect(), as it's not possible to add
comments to specific lines of the case construct anyways.

Fixes: 9a5801e7 "ar71xx: add model detection for UBNT Rocket Ti"
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>