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_9.txt

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