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_1.txt

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