Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/tui/src/skills_helpers.rs

@@ -1,43 +0,0 @@
-use codex_core::skills::model::SkillMetadata;
-use codex_utils_fuzzy_match::fuzzy_match;
-
-use crate::text_formatting::truncate_text;
-
-pub(crate) const SKILL_NAME_TRUNCATE_LEN: usize = 21;
-
-pub(crate) fn skill_display_name(skill: &SkillMetadata) -> &str {
-    skill
-        .interface
-        .as_ref()
-        .and_then(|interface| interface.display_name.as_deref())
-        .unwrap_or(&skill.name)
-}
-
-pub(crate) fn skill_description(skill: &SkillMetadata) -> &str {
-    skill
-        .interface
-        .as_ref()
-        .and_then(|interface| interface.short_description.as_deref())
-        .or(skill.short_description.as_deref())
-        .unwrap_or(&skill.description)
-}
-
-pub(crate) fn truncate_skill_name(name: &str) -> String {
-    truncate_text(name, SKILL_NAME_TRUNCATE_LEN)
-}
-
-pub(crate) fn match_skill(
-    filter: &str,
-    display_name: &str,
-    skill_name: &str,
-) -> Option<(Option<Vec<usize>>, i32)> {
-    if let Some((indices, score)) = fuzzy_match(display_name, filter) {
-        return Some((Some(indices), score));
-    }
-    if display_name != skill_name
-        && let Some((_indices, score)) = fuzzy_match(skill_name, filter)
-    {
-        return Some((None, score));
-    }
-    None
-}