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_8.txt

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