um: Cleanup CONFIG_IOSCHED_CFQ

CONFIG_IOSCHED_CFQ is gone since commit f382fb0b ("block: remove legacy
IO schedulers").

The IOSCHED_BFQ seems to replace IOSCHED_CFQ so select it in configs
previously choosing the latter.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>