ath9k: debug: remove set but not used variable 'fops_dump_nfcal'

'fops_dump_nfcal' is not used since commit 4447d815 ("ath: ath9k: use debugfs_create_devm_seqfile() helper for seq_file entries")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>