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/feature_flags/src/feature_flags.rs

@@ -43,11 +43,6 @@ impl FeatureFlag for LanguageModels {
     const NAME: &'static str = "language-models";
 }
 
-pub struct TerminalInlineAssist {}
-impl FeatureFlag for TerminalInlineAssist {
-    const NAME: &'static str = "terminal-inline-assist";
-}
-
 pub struct GroupedDiagnostics {}
 impl FeatureFlag for GroupedDiagnostics {
     const NAME: &'static str = "grouped-diagnostics";