assistant: Unship tool use (#23969)

This PR unships tool use from Assistant1.

This was only ever partially implemented, and was never released to end
users.

Assistant2 will support tool use.

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -41,16 +41,6 @@ impl FeatureFlag for Assistant2FeatureFlag {
     const NAME: &'static str = "assistant2";
 }
 
-pub struct ToolUseFeatureFlag;
-
-impl FeatureFlag for ToolUseFeatureFlag {
-    const NAME: &'static str = "assistant-tool-use";
-
-    fn enabled_for_staff() -> bool {
-        false
-    }
-}
-
 pub struct PredictEditsFeatureFlag;
 impl FeatureFlag for PredictEditsFeatureFlag {
     const NAME: &'static str = "predict-edits";