HID: intel-ish-hid: Remove unused ishtp_cl_get_tx_*

ishtp_cl_get_tx_free_buffer_size() and ishtp_cl_get_tx_free_rings()
were added in 2018 by
commit 18c0b546 ("HID: intel_ish-hid: Enhance API to get ring buffer
sizes")
but have remained unused.

Remove them.

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>