onboarding: Remove feature flag and old welcome crate (#36110)

Release Notes:

- N/A

---------

Co-authored-by: MrSubidubi <dev@bahn.sh>
Co-authored-by: Anthony <anthony@zed.dev>

crates/zed/Cargo.toml

@@ -157,7 +157,6 @@ vim_mode_setting.workspace = true
 watch.workspace = true
 web_search.workspace = true
 web_search_providers.workspace = true
-welcome.workspace = true
 workspace-hack.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true