fish: remove promptInit in favor of interactiveShellInit

[kidonng]

Description

promptInit does nothing special and can already be implemented via interactiveShellInit, so I suggest just keep the latter.

However there are some things I'm not too sure:

• This may break some people's config. I don't know the process of deprecating an option in Home Manager, so help welcome.
• There is already a programs.fish.interactiveShellInit (which "add completions generated by Home Manager to $fish_complete_path") config, but is it a default value? Will it take effect if users provide a custom interactiveShellInit? If that's the case then it should be fixed (and the code looks horrible to me, so we may also have a chance to rewrite it at the same time).

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

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[kidonng]

Bump, this is still relevant.

[berbiche]

LGTM I guess