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_2.txt

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