title_bar: Simplify git-ui feature flag check (#23475)

This PR is a follow-up to
https://github.com/zed-industries/zed/pull/23470 that simplifies the way
we check the `git-ui` feature flag in the title bar.

Release Notes:

- N/A

Cargo.lock

@@ -5242,7 +5242,6 @@ dependencies = [
  "collections",
  "db",
  "editor",
- "feature_flags",
  "futures 0.3.31",
  "git",
  "gpui",
@@ -5254,7 +5253,6 @@ dependencies = [
  "serde_derive",
  "serde_json",
  "settings",
- "smol",
  "theme",
  "ui",
  "util",