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>

Cargo.toml

@@ -185,7 +185,6 @@ members = [
     "crates/watch",
     "crates/web_search",
     "crates/web_search_providers",
-    "crates/welcome",
     "crates/workspace",
     "crates/worktree",
     "crates/x_ai",
@@ -412,7 +411,6 @@ vim_mode_setting = { path = "crates/vim_mode_setting" }
 watch = { path = "crates/watch" }
 web_search = { path = "crates/web_search" }
 web_search_providers = { path = "crates/web_search_providers" }
-welcome = { path = "crates/welcome" }
 workspace = { path = "crates/workspace" }
 worktree = { path = "crates/worktree" }
 x_ai = { path = "crates/x_ai" }