ath10k: remove deprecated fw_crash_dump debugfs file

The fw_crash_dump file was deprecated by commmit 727000e6

 ("ath10k: support
dev_coredump for crash dump") in v4.11 in favor of dev_coredump interface,
remove it now for good. Everyone should use dev_coredump now.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>