staging: iio: remove iio-trig-bfin-timer driver

The blackfin architecture is getting removed, so the timer trigger
driver is now obsolete. Since this is the last remaining iio trigger
driver in staging, I'm removing the entire directory.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Aaron Wu <aaron.wu@analog.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>