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_34.txt

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