Removes `diagnostics.cargo.fetch_cargo_diagnostics` settings as those
are not needed for the flycheck diagnostics to run.
This setting disabled `checkOnSave` in rust-analyzer and allowed to
update diagnostics via flycheck in the project diagnostics editor with
the "refresh" button.

Instead, `"checkOnSave": false,` can be set manually as
https://zed.dev/docs/languages/rust#more-server-configuration example
shows and flycheck commands can be called manually from anywhere,
including the diagnostics panel, to refresh the diagnostics.

Release Notes:

- Removed redundant `diagnostics.cargo.fetch_cargo_diagnostics` settings