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_20.txt

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