rt2x00: Remove RT2870 chipset identification.

There is no evidence, either in adapters or in the Ralink code, that such
a device actually exists. All so-call RT2870 adapter identify themselves
as RT2860.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>