completions/cargo: remove xtask completions for now

We source "COMPLETE=fish cargo xtask" at completion load time.

This means that completions will be missing if completions were requested 
before entering a fish-shell worktree.

Also, as reported in [1], this will compile xtask even for non-fish
projects, which is usually fast but not necessarily (it might download
crates etc).

As suggested in the initial revision of #12763, we could
hardcode the output of

	$ COMPLETE=fish cargo xtask
	complete --keep-order --exclusive --command cargo --arguments \
		"(COMPLETE=fish /home/johannes/git/fish-shell/target/debug/xtask --
			(commandline --current-process --tokenize --cut-at-cursor) (commandline --current-token))"

but that might still be too intrusive because according to [2],
calls to "cargo metadata" can be slow on big repos with hundreds of
Cargo.toml files.

[1]: https://github.com/fish-shell/fish-shell/commit/959cbb42597444577d26ed5ba6151ccb1d7f3600#r185615416
[2]: https://github.com/fish-shell/fish-shell/commit/959cbb42597444577d26ed5ba6151ccb1d7f3600#r185637424

FWIW, commit comments are also accessible via

	gh api -H "X-GitHub-Api-Version: 2026-03-10" \
	    /repos/fish-shell/fish-shell/commits/959cbb42597444577d26ed5ba6151ccb1d7f3600/comments