Since the `fish_add_path` command modifies a universal Fish variable
(which is automatically persisted to a file) it's unnecessary to run it
again every time a new shell is opened.
fixes https://github.com/Homebrew/brew/issues/8965
See Fish docs https://fishshell.com/docs/current/tutorial.html#path.
> A faster way is to use the `fish_add_path function`... The advantage is that
> you don't have to go mucking around in files: just run this once at the
> command line, and it will affect the current session and all future instances
> too.
>
> Or you can modify `$fish_user_paths` yourself, but you should be careful not
> to append to it unconditionally in config.fish, or it will grow longer and
> longer.
Deprecate more methods. Internal APIs have been verified to be unused
elsewhere and removed. External APIs have had deprecation methods added.
Existing deprecations have been either upgraded to produce warnings or
no longer deprecated and the reasoning documented.
