HID: i2c-hid: remove orphaned member sleep_delay

This was introduced in commit 00b790ea ("HID: i2c-hid: Add a small
delay after sleep command for Raydium touchpanel") which has been
effectively reverted by commit 67b18dfb ("HID: i2c-hid: Remove
runtime power management").

Signed-off-by: You-Sheng Yang <vicamo.yang@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>