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/frames/dots/frame_7.txt

@@ -1,17 +0,0 @@
-                                      
-             ○◉·◉●●○○●●○              
-          ●●·●○·●●●○○○◉ ·●●           
-         ● ●···●·  ·○·· ●◉○●●         
-        ○○····◉●       ○·○●·◉●        
-       ·●·●··○·◉○●       ◉ ●··○       
-      · ◉··  ·●○◉·○       ●◉···       
-       ·○ ·  ○○·○·○·       ●●○··      
-      ○ ◉··   ○●◉·●··      ● ···      
-       ◉ ·● ● ○◉·○◉··◉◉○··○◉●○··      
-      ·●●··○◉··◉●·◉○··○○●○●◉○·○·      
-       ○ ○○◉●·●○◉  ●●●●●●  ○◉··       
-       ●○ ◉○◉○·         ● ◉◉◉◉●       
-        ●◉●●○○○○      ●○·●●◉●●        
-          ○○●○·●○◉·○○○●●·●●◉          
-             ·● ·◉·○○●○◉●●            
-