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/src/assistant_panel.rs

@@ -644,11 +644,6 @@ impl AssistantPanel {
                 .focus_handle(cx)
                 .contains_focused(cx)
             {
-                use feature_flags::FeatureFlagAppExt;
-                if !cx.has_flag::<feature_flags::TerminalInlineAssist>() {
-                    return None;
-                }
-
                 if let Some(terminal_view) = terminal_panel
                     .read(cx)
                     .pane()