cgroup: Add missing errno == ENOENT check in virCgroupRemoveRecursively

cgroup: Add missing errno == ENOENT check in virCgroupRemoveRecursively

ENOENT happens normally when a subsystem is enabled with any other
subsystems and the directory of the target group has already removed
in a prior loop. In that case, the function should just return without
leaving an error message.

NB this is the same behavior as before introducing virCgroupRemoveRecursively.