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_18.txt

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