Remove redundant Cargo diagnostics settings (#36795)

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

assets/settings/default.json

@@ -1133,11 +1133,6 @@
       // The minimum severity of the diagnostics to show inline.
       // Inherits editor's diagnostics' max severity settings when `null`.
       "max_severity": null
-    },
-    "cargo": {
-      // When enabled, Zed disables rust-analyzer's check on save and starts to query
-      // Cargo diagnostics separately.
-      "fetch_cargo_diagnostics": false
     }
   },
   // Files or globs of files that will be excluded by Zed entirely. They will be skipped during file