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_32.txt

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