assistant: Remove feature flag for terminal inline assistant (#15432)

This PR removes the feature flag for the terminal inline assistant,
making it available to everyone.

Release Notes:

- Use Ctrl+Enter to summon the inline assistant in the terminal, which
let's you generate terminal commands based on your description
([demo](https://twitter.com/thorstenball/status/1814241447383605329))

crates/assistant/Cargo.toml

@@ -34,7 +34,6 @@ collections.workspace = true
 command_palette_hooks.workspace = true
 completion.workspace = true
 editor.workspace = true
-feature_flags.workspace = true
 fs.workspace = true
 futures.workspace = true
 fuzzy.workspace = true