Clean up inline assist editor rendering (#15536)

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/diagnostics/Cargo.toml

@@ -18,13 +18,11 @@ collections.workspace = true
 ctor.workspace = true
 editor.workspace = true
 env_logger.workspace = true
-feature_flags.workspace = true
 futures.workspace = true
 gpui.workspace = true
 language.workspace = true
 log.workspace = true
 lsp.workspace = true
-multi_buffer.workspace = true
 project.workspace = true
 rand.workspace = true
 schemars.workspace = true