ath79: ar9344_mikrotik_routerboard: drop runnig LED alias

[Thibaut VARÈNE]

From: Thibaut VARENE <hacks at slashdirt.org>

Please don't do this.
The rationale has been already explained here:
https://github.com/openwrt/openwrt/pull/10135#pullrequestreview-1020528516

The gist of it is: the power LED alone cannot disambiguate a powered-on
device from a properly operating one. It also breaks "established standard"
on all other mikrotik devices.

T.