HID: elo: clear BTN_LEFT mapping

ELO devices have one Button usage in GenDesk field, which makes hid-input map
it to BTN_LEFT; that confuses userspace, which then considers the device to be
a mouse/touchpad instead of touchscreen.

Fix that by unmapping BTN_LEFT and keeping only BTN_TOUCH in place.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>