remove xmpp support

I've long considered the XMPP support in git-annex a wart.
It's nice to remove it.

(This also removes the NetMessager, which was only used for XMPP, and the
daemonstatus's desynced list (likewise).)

Existing XMPP remotes should be ignored by git-annex.

This commit was sponsored by Brock Spratlen on Patreon.

doc/assistant/share_with_a_friend_walkthrough.mdwn

@@ -1,58 +0,0 @@
-Want to share all the files in your repository with a friend?
-
-Let's suppose you use Google Mail, and so does your friend, and you
-sometimes also chat in Google Talk. The git-annex assistant will
-use your Google account to share with your friend. (This actually
-works with any Jabber account you use, not just Google Talk.)
-
-Start by opening up your git annex dashboard.
-
-[[!img local_pairing_walkthrough/addrepository.png alt="Add another repository button"]]
-
-`*click*`
-
-[[!img pairing.png alt="Share with a friend"]]
-
-`*click*`
-
-[[!img xmpp.png alt="Configuring Jabber account"]]
-
-Fill that out, and git-annex will be able to show you a list of your
-friends.
-
-[[!img buddylist.png alt="Buddy list"]]
-
-This list will refresh as friends log on and off, so you can
-leave it open in a tab until a friend is available to start pairing.
-
-(If your friend is not using git-annex yet, now's a great time to spread
-the word!)
-
-Once you click on "Start Pairing", your friend will see this pop up
-on their git annex dashboard.
-
-[[!img xmppalert.png alt="Pair request"]]
-
-Once your friend clicks on that, your repositories will be paired.
-
-### But, wait, there's one more step...
-
-Despite the repositories being paired now, you and your friend can't yet
-quite share files. You'll start to see your friend's files show up in your
-git-annex folder, but you won't be able to open them yet.
-
-What you need to do now is set up a repository out there in the cloud,
-that both you and your friend can access. This will be used to transfer
-files between the two of you.
-
-At the end of the pairing process, a number of cloud providers are
-suggested, and the git-annex assistant makes it easy to configure one of
-them. Once you or your friend sets it up, it'll show up in the other
-one's list of repositories:
-
-[[!img repolist.png alt="Repository list"]]
-
-The final step is to share the login information for the cloud repository
-with your friend, so they can enable it too.
-
-With that complete, you'll be able to open your friend's files!