Revert "ath79-generic: (re)add Rocket M2/M5 (XM)"

This reverts commit 97efc8ab. (#2580)

Since the commit was openend the device was moved from ath79-generic to ath79-tiny ([4b5bd15](https://github.com/openwrt/openwrt/commit/4b5bd1509195bc8f19999ebe481b59356b5c3512)).

Therefore the commit breaks builds of the ath79-generic target (`unable to enable device 'ubnt_rocket-m'`).

It might be possible to add support for the ath79-tiny target, and with it the Rocket M2/M5 (XM), in the future.