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_3.txt

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