cgroup-util: drop handcrafted cg_is_empty(), always check cgroup.events populated...

cgroup-util: drop handcrafted cg_is_empty(), always check cgroup.events populated field

This effectively renames cg_is_empty_recursive() to cg_is_empty().
Note that all existing code calls the former and not the latter,
hence with cgv1 support being dropped it's trivial to consult
cgroup.events directly for populated state everywhere.

Additionally, use more generic cg_get_keyed_attribute() helper
rather than cg_read_event().