btrfs: remove fs_info::pending_changes and related code

Now that we're not using this code anywhere we can remove it as well as
the member from fs_info.

We don't have any mount options or on/off features that would utilize
the pending infrastructure, the last one was inode_cache.
There was a patchset [1] to enable some features from sysfs that would
break things if it would be set immediately. In case we'll need that
kind of logic again the patch can be reverted, but for the current use
it can be replaced by the single state bit to do the commit.

[1] https://lore.kernel.org/linux-btrfs/1422609654-19519-1-git-send-email-quwenruo@cn.fujitsu.com/



Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add note ]
Signed-off-by: David Sterba <dsterba@suse.com>