btrfs: Remove unused btrfs_start_transaction_lflush function

Commit 0e8c36a9 ("Btrfs: fix lots of orphan inodes when the space
is not enough") changed the way transaction reservation is made in
btrfs_evict_node and as a result this function became unused. This has
been the status quo for 5 years in which time no one noticed, so I'd
say it's safe to assume it's unlikely it will ever be used again.

Historical note: there were more attempts to remove the function, the
reasoning was missing and only based on some static analysis tool
reports. Other reason for rejection was that there seemed to be
connection to BTRFS_RESERVE_FLUSH_LIMIT and that would need to be
removeed to. This was not correct so removing the function is all we can
do.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
[ add the note ]
Signed-off-by: David Sterba <dsterba@suse.com>