rt2x00: remove the incompatible 20 mhz clock patch again (fixes #13532)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@39952 3c298f89-4303-0410-b956-a3cf2f4a3e73