Btrfs: remove unused mutex from struct 'btrfs_fs_info'

The code using 'ordered_extent_flush_mutex' mutex has removed by below
commit.
 - 8d875f95


   btrfs: disable strict file flushes for renames and truncates
But the mutex still lives in struct 'btrfs_fs_info'.

So, this patch removes the mutex from struct 'btrfs_fs_info' and its
initialization code.

Signed-off-by: Byongho Lee <bhlee.kernel@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>