Remove assistant hints (#21171)

This reverts #20824 and #20899. After adding them last week we came to
the conclusion that the hints are too distracting in everyday use, see
#21128 for more details.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -1,5 +1,4 @@
 mod app_menus;
-pub mod assistant_hints;
 pub mod inline_completion_registry;
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
 pub(crate) mod linux_prompts;