How can I remove the Mac recovery partition from an external drive?
Your disk1 and disk3 are the virtual disks created by CoreStorage, that is, the system underlying FileVault 2 (full disk encryption). This is why your data partitions on the "real" disks (disk0 and disk2) have type Apple_CoreStorage.

The recovery partition on your external drive is this one:

   3:                 Apple_Boot Recovery HD             650.0 MB   disk2s3


Deleting it may however not be a good idea. Because CoreStorage drives are encrypted, the bootloader can't read any data from them - the OSX kernel (with the CoreStorage driver) needs to be loaded to ask for the user password, which opens up the main drive containing the rest of the operating system and data. So with CoreStorage enabled, the Recovery HD partition doubles as an unencrypted boot helper.

I don't have much experience with CoreStorage, and you haven't made it completely clear what your backup disk contains. I know that unencrypted Time Machine disks are bootable. I would imagine that encrypted ones are, too, with the caveat of requiring a boot helper, but I don't know for sure. Furthermore, since you only mention "backups", maybe you're using a different backup system which may or may not be bootable.

If your backups are not intended to be bootable, I think you should be able to delete the Recovery HD, although there is a certain risk that CoreStorage relies on it nevertheless. If they are bootable, deleting it outright will make them fail to boot. You could try writing the partition parameters displayed by gpt show disk2, deleting the partition using gpt without overwriting it, rebooting, and checking that it still works, and recreating it with the exact parameters (again using gpt, not diskutil) if you have trouble accessing your backups.

However, boot helpers don't actually need to contain a full recovery system. Prior to OSX Lion, boot helpers (e.g. used for software RAID) were always 128MiB in size, and this is still enough for Lion and Mountain Lion. Still, recreating a deleted boot helper isn't exactly straightforward.

Then there's the question of what you'll do with the extra ~520MB you gained. I don't think you can resize core storage partitions, so all you'll be able to do is create a ~400MB additional partition, which isn't terribly useful.

Overall, I'd say it's not worth the risk/trouble to gain ~500MB.