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_22.txt

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