Remove vg join (been deprecated since 2020)

[faithokamoto]

Changelog Entry

To be copied to the draft changelog by merger:

• vg join is removed

Description

Another of the subcommands we deprecated back in 2020 with #3136. vg join is problematic: #2515 (comment) (comment from 2019):

We really need to remove vg join because it does not do what users expect!
It links the graphs to a single head node, and was used during testing I
did five years ago. This is getting urgent because we are having one to two
support requests a week about this.

That issue thread also suggests removing vg concat, which also does unexpected things #2502 (comment) -- except vg concat does some useful things too, e.g. see it being recommended here #2688. So I am not sure whether it should be murdered with fire. Thus this PR only removes vg join.

[faithokamoto]

Also, you can do exactly what vg join is for by just manipulating GFAs. I did that for Centrolign and it worked fine.