Remove experimental jj UI, for now (#38743)

This PR removes the experimental jj bookmark picker that was added in
#30883.

This was just an exploratory prototype and while I would like to have
native jj UI at some point, I don't know when we'll get back to it.

Release Notes:

- N/A

crates/feature_flags/src/flags.rs

@@ -27,9 +27,3 @@ pub struct PanicFeatureFlag;
 impl FeatureFlag for PanicFeatureFlag {
     const NAME: &'static str = "panic";
 }
-
-pub struct JjUiFeatureFlag {}
-
-impl FeatureFlag for JjUiFeatureFlag {
-    const NAME: &'static str = "jj-ui";
-}