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_33.txt

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