protocol: Remove wl_ prefix from summary descriptions

Summary attributes sometime describe objects using their wl-prefixed type,
but more often don't.

Remove the wl_ prefix from summary descriptions, since they tend to describe
concepts.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>