Remove the view command and the editor setting with it

`view` was the last user of `editor`, so leaving the setting behind would
have made it exactly what `gitkey` was until recently: documented,
parsed, and doing nothing. It is gone from the struct, the template, the
environment, `config -k` and the settings table.

Both names are free for aliases now, as `open` already was.

The tests that used `editor` as their example of a project-overridable
setting use `gitkey` instead, which is the same kind of thing and still
exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

mgshrc.example

@@ -20,7 +20,6 @@ gitpath     = /home/git
 # gitname     = Your Name
 # gitemail    = you@example.com
 # pushdefault = matching
-# editor      = code
 
 # --- pushremote: mirror to public servers (gitea/github/gitlab) via their API ---
 # One "remote.<name>.<field>" block per server, with the fields url, key, type