btrfs: remove useless comment from btrfs_pin_extent_for_log_replay()

The comment on top of btrfs_pin_extent_for_log_replay() mentioning that
the function must be called within a transaction is pointless as of
commit 9fce5704 ("btrfs: Make btrfs_pin_extent_for_log_replay take
transaction handle"), since the function now takes a transaction handle
as its first argument. So remove the comment because it's completely
useless now.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>