language_models: Remove language-models feature flag (#29416)

This PR removes the `language-models` feature flag.

This feature is already generally available, so we no longer need the
feature flag.

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -64,11 +64,6 @@ impl FeatureFlag for PredictEditsRateCompletionsFeatureFlag {
     const NAME: &'static str = "predict-edits-rate-completions";
 }
 
-pub struct LanguageModelsFeatureFlag {}
-impl FeatureFlag for LanguageModelsFeatureFlag {
-    const NAME: &'static str = "language-models";
-}
-
 pub struct LlmClosedBetaFeatureFlag {}
 impl FeatureFlag for LlmClosedBetaFeatureFlag {
     const NAME: &'static str = "llm-closed-beta";