HID: intel-ish-hid: Remove unused ishtp_cl_tx_empty

ishtp_cl_tx_empty() was added in 2018 by
commit a1c40ce6 ("HID: intel-ish-hid: ishtp: add helper functions for
client buffer operation") but has remained unused.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>