Cleanup dependencies (part 4) (#8468)

Follow-up to #8425 . Final part - adds the CI check.

Release Notes:
- N/A

crates/diagnostics/Cargo.toml

@@ -18,13 +18,10 @@ gpui.workspace = true
 language.workspace = true
 log.workspace = true
 lsp.workspace = true
-postage.workspace = true
 project.workspace = true
 schemars.workspace = true
 serde.workspace = true
-serde_derive.workspace = true
 settings.workspace = true
-smallvec.workspace = true
 theme.workspace = true
 ui.workspace = true
 util.workspace = true