ath10k: sdio: remove unused struct member

irq_wq in struct ath10k_sdio is a remnant from an earlier
version of the sdio patchset.

Its use was removed as a result of Kalle's review, but somehow
the struct member survived.

It is not used and can therefore safely be removed.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>