avr32: remove fake at91 cpu identification

cpu_is_at91* are not used in any driver outside mach-at91. Remove those
useless definitions.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>