vim: Remove ctrl-6 keybinding alias for pane::AlternateFile (#44560)

Closes #ISSUE

It seems that `ctrl-6` is used exclusively as an alias, as can be seen
in the [linked section of the vim
docs](https://vimhelp.org/editing.txt.html#CTRL-%5E) from the initial PR
that added it. This however conflicts with the `ctrl-{n}` bindings for
`pane::ActivateItem` on macOS, leading to confusing file selection when
`ctrl-6` is pressed.

Release Notes:

- vim(BREAKING): Removed a keybinding conflict between the default macOS
bindings for `pane::ActivateItem` and the `ctrl-6` alias
for`pane::AlternateFile` which is primarily bound to `ctrl-^`. `ctrl-6`
is no longer treated as an alias for `ctrl-^` in vim mode. If you'd like
to restore `ctrl-6` as a binding for `pane::AlternateFile`, paste the
following into your `keymap.json` file:
```
  {
    "context": "VimControl && !menu",
    "bindings": {
      "ctrl-6": "pane::AlternateFile"
    }
  }
```

assets/keymaps/vim.json

@@ -180,7 +180,6 @@
       "ctrl-w g shift-d": "editor::GoToTypeDefinitionSplit",
       "ctrl-w space": "editor::OpenExcerptsSplit",
       "ctrl-w g space": "editor::OpenExcerptsSplit",
-      "ctrl-6": "pane::AlternateFile",
       "ctrl-^": "pane::AlternateFile",
       ".": "vim::Repeat"
     }