HID: intel-ish-hid: Remove unused declarations

Commit 3703f53b ("HID: intel_ish-hid: ISH Transport layer")
declared ishtp_remove_all_clients()/ishtp_can_client_connect()
but never implemented them.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>