Remove the `reaction pull` command

[brent-hoover]

This command doesn't work unless you have a very specific configuration so it causes a lot of confusion and doesn't add a ton of value. I would suggest just removing it.

[prinzdezibel]

Lately there was a related question on Gitter/Forum again. She/he couldn't get it work. I guess because their setup did not match the "canonical" way we suppose it to be.

[duongvu89]

There should be a way to update new version of Reaction either by reaction pull or anything else. Could you please add the topic in the tutorial?