btrfs: Remove unused flush var in shrink_delalloc

Added by 08e007d2 ("Btrfs: improve the noflush reservation") and
made redundant by 17024ad0

 ("Btrfs: fix early ENOSPC due to
delalloc").

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>