bug#43138: Stack overflow in emacs 27 because of preloading emacs-seq

[Pierre Langlois]

Hello Guix!

Since switching to emacs 27 I've been having issues starting it, seeing
lots of errors like 'Lisp nesting exceeds ‘max-lisp-eval-depth’' when
loading various packages such as magit, ivy, ...etc.

After quite a bit of troubleshooting I reduced it to the `emacs-seq`
package. So if you create an environment with both `emacs-seq` and say
`magit`:

```
$ guix environment --pure --ad-hoc emacs emacs-magit emacs-seq
[env] $ emacs -Q --debug-init --eval "(require 'magit)"
```

Then you get the stack overflow.

Doing some digging, I found this comment from `doom-emacs` that
describes what is happening: 
https://github.com/hlissner/doom-emacs/issues/1990#issuecomment-549140718

I'm not familiar with emacs' autoloading, so I'm not sure I understand
what's going on fully. However, it mentions that `emacs-seq` has been
included in emacs proper for a while.

So, what would be the best fix for this? Should we remove `emacs-seq`
entirely or try and patch it? Since we don't support previous versions
of emacs I don't know if we need it.

Thanks,
Pierre