btrfs: remove BLOCK_GROUP_FLAG_HAS_CACHING_CTL

This is used mostly to determine if we need to look at the caching ctl
list and clean up any references to this block group.  However we never
clear this flag, specifically because we need to know if we have to
remove a caching ctl we have for this block group still.  This is in the
remove block group path which isn't a fast path, so the optimization
doesn't really matter, simplify this logic and remove the flag.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>